Email has its own language. SPF, DKIM, DMARC, IMAP, MX records, bounce rates, warm-up, throttling — whether you’re an IT admin configuring authentication, a marketer optimizing deliverability, or a professional trying to understand why your emails land in spam, this glossary covers every term you need to know. All 100+ definitions are written in plain language with practical context.

How to Use This Glossary

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A

A Record Protocol
A DNS record that maps a domain name to an IPv4 address. While not email-specific, A records are used in email authentication and server resolution. See also: MX Record.
AI Email Assistant AI
Software that uses artificial intelligence to help manage email — drafting replies, summarizing threads, extracting action items, categorizing messages, or filtering noise. Modern AI email assistants like Unboxd use large language models to understand email content and reduce inbox time by up to 90%.
Alias General
An alternative email address that forwards to your primary inbox. For example, [email protected] might be an alias that delivers to [email protected]. Aliases don’t require separate mailboxes.
Attachment General
A file sent alongside an email message. Common attachment types include documents (PDF, DOCX), images (PNG, JPG), and spreadsheets (XLSX). Most email providers limit attachment size to 25 MB, with larger files typically shared via cloud links.
Authentication Security
The process of verifying that an email actually came from the domain it claims to be from. The three main email authentication protocols are SPF, DKIM, and DMARC. Since February 2024, Google and Yahoo require authentication for all bulk senders.
Autoresponder General
An automatic reply sent when someone emails you. The most common example is an out-of-office message. Autoresponders can also be used in marketing for welcome sequences or confirmation emails.

B

BCC (Blind Carbon Copy) General
A field in email composition that sends a copy to recipients without revealing their addresses to other recipients. Use BCC for mass emails to protect privacy, or when you need to loop someone in discreetly. See also: CC.
BIMI (Brand Indicators for Message Identification) Deliverability
An email standard that displays a brand’s logo next to authenticated emails in the recipient’s inbox. BIMI requires a valid DMARC policy set to quarantine or reject. It increases brand visibility and recipient trust. Check yours with our BIMI checker.
Blacklist (Blocklist) Deliverability
A database of IP addresses or domains identified as sources of spam. If your sending IP appears on a blacklist, your emails may be rejected or sent to spam. Major blacklists include Spamhaus, Barracuda, and SpamCop. Check yours with our blacklist checker.
Bounce Deliverability
When an email cannot be delivered and is returned to the sender. A hard bounce means the address doesn’t exist (permanent failure). A soft bounce means a temporary issue like a full mailbox or server downtime. High bounce rates damage your sender reputation.
Bounce Rate Marketing
The percentage of sent emails that bounce. A healthy bounce rate is below 2%. Rates above 5% signal list quality issues and can trigger blacklisting. Calculate it as: (bounced emails / total sent) × 100.
Bulk Sender Deliverability
An entity that sends large volumes of email, typically defined by Google as anyone sending 5,000+ messages per day to Gmail addresses. Bulk senders must comply with stricter authentication requirements (SPF, DKIM, DMARC) since February 2024. Check compliance with our bulk sender checker.

C

CC (Carbon Copy) General
A field in email composition that sends a copy to additional recipients. Unlike BCC, all recipients can see who was CC’d. Use CC to keep stakeholders informed without making them primary recipients. See also: BCC.
Click-Through Rate (CTR) Marketing
The percentage of email recipients who clicked a link within the email. Calculated as (unique clicks / delivered emails) × 100. Average email CTR across industries is approximately 2–3% (Source: Mailchimp 2025).
CNAME Record Protocol
A DNS record that creates an alias from one domain name to another. CNAME records are commonly used in email for DKIM verification, pointing a selector subdomain to your email provider’s DKIM key.
Cold Email Marketing
An unsolicited email sent to a recipient with no prior relationship. Cold emails are common in sales and recruiting. Effective cold emails are personalized, concise, and comply with anti-spam laws like CAN-SPAM and GDPR.
Complaint Rate Deliverability
The percentage of recipients who mark your email as spam. Google requires bulk senders to maintain a complaint rate below 0.3%. A rate above 0.1% is already a warning sign. Calculated as (spam complaints / delivered emails) × 100.

D

Dedicated IP Deliverability
An IP address used exclusively by one sender for sending email. Unlike a shared IP (used by multiple senders), a dedicated IP means your sender reputation depends entirely on your own sending practices. Recommended for senders doing 100K+ emails per month.
Deliverability Deliverability
The ability of an email to reach the recipient’s inbox (not spam folder or rejection). Deliverability depends on sender reputation, authentication (SPF/DKIM/DMARC), content quality, and list hygiene. Test yours with our deliverability tester.
DKIM (DomainKeys Identified Mail) Security
An email authentication method that adds a digital signature to outgoing emails. The sending server signs emails with a private key; receiving servers verify the signature using a public key published in DNS. DKIM proves the email wasn’t altered in transit. Check yours with our DKIM checker.
DMARC (Domain-based Message Authentication, Reporting, and Conformance) Security
An email authentication policy built on top of SPF and DKIM. DMARC tells receiving servers what to do when an email fails authentication: none (monitor only), quarantine (send to spam), or reject (block entirely). It also provides aggregate reports on authentication results. Check yours with our DMARC analyzer.
DNS (Domain Name System) Protocol
The internet’s phone book — translates domain names (like unboxd.ai) into IP addresses. Email relies heavily on DNS for MX records (where to deliver), SPF records (who can send), DKIM records (signature verification), and DMARC records (authentication policy).
Domain Reputation Deliverability
A score assigned to your sending domain by email providers based on your sending history. Factors include bounce rates, spam complaints, authentication setup, and engagement metrics. A poor domain reputation causes emails to land in spam regardless of content.
Double Opt-In Marketing
A subscription method where new subscribers must confirm their email address by clicking a link in a confirmation email. Double opt-in produces higher-quality lists with lower bounce and complaint rates compared to single opt-in.
Drip Campaign Marketing
A series of automated emails sent on a schedule or triggered by user actions. Common drip campaigns include welcome sequences, onboarding flows, and re-engagement series. Each email builds on the previous one to nurture the recipient.

E

Email Authentication Security
The collective term for protocols (SPF, DKIM, DMARC) that verify an email’s sender identity. Proper authentication prevents spoofing, improves deliverability, and is required by major providers since 2024. Use our authentication setup wizard to configure all three.
Email Client General
The application used to read, compose, and manage email. Examples include Gmail (web), Apple Mail (desktop), Outlook (desktop/web), and Unboxd (AI-powered). Email clients connect to mail servers via IMAP, POP3, or proprietary APIs.
Email Encryption Security
The process of encoding email content so only authorized recipients can read it. Types include TLS (encrypts in transit between servers), S/MIME (end-to-end with certificates), and PGP (end-to-end with key pairs). Unboxd uses AES-256-GCM encryption with per-user keys for stored email data.
Email Forwarding General
Sending a received email to another recipient. Forwarding can break DKIM signatures if the forwarding server modifies headers. See also: ARC.
Email Gateway Protocol
A server that processes email before it reaches the recipient’s mail server. Gateways typically handle spam filtering, virus scanning, encryption, and policy enforcement. Common in enterprise environments (e.g., Proofpoint, Mimecast).
Email Header Protocol
Metadata at the top of every email message containing routing information, timestamps, authentication results, and sender/recipient details. Headers are normally hidden from view but are essential for troubleshooting delivery issues. Analyze yours with our header analyzer.
Envelope Sender (Return-Path) Protocol
The email address used in the SMTP MAIL FROM command, which receives bounce notifications. This can differ from the visible “From” address in the email header. SPF checks are performed against the envelope sender domain.
ESP (Email Service Provider) Marketing
A platform for sending marketing and transactional emails at scale. Examples include Mailchimp, SendGrid, Postmark, and Amazon SES. ESPs provide sending infrastructure, list management, analytics, and compliance features.

F

Feedback Loop (FBL) Deliverability
A service offered by ISPs that forwards spam complaints back to senders. When a recipient marks your email as spam, the FBL notifies you so you can remove them from your list. Major FBLs include Gmail Postmaster Tools and Microsoft SNDS.
Filtering General
Rules that automatically sort incoming emails into folders, labels, or categories. Traditional filters use static rules (sender, subject, keywords). AI-powered filtering uses natural language understanding to categorize by content and intent.
From Header Protocol
The visible sender address displayed to the recipient. This can differ from the envelope sender (Return-Path). DMARC alignment checks whether the From header domain matches the SPF or DKIM domain.

G

Graymail General
Email that isn’t spam but isn’t wanted either — newsletters you subscribed to but never read, promotional emails from legitimate companies, automated notifications. Graymail is the largest contributor to inbox overload and is what AI email tools like Unboxd categorize as “FYIs.”
Greylisting Deliverability
An anti-spam technique where a mail server temporarily rejects emails from unknown senders. Legitimate servers retry delivery (and succeed); spammers typically don’t retry. Greylisting can delay first-time emails by several minutes.

H

Hard Bounce Deliverability
A permanent email delivery failure, usually because the recipient address doesn’t exist. Hard bounces should be immediately removed from your list. A hard bounce rate above 2% signals poor list hygiene. See also: Soft Bounce.
HTML Email General
An email formatted with HTML and CSS, allowing rich content like images, buttons, colors, and layouts. Most marketing emails are HTML. Preview how yours renders across clients with our HTML email previewer.

I

IMAP (Internet Message Access Protocol) Protocol
A protocol for accessing email on a remote server. IMAP keeps emails on the server and syncs state across multiple devices — read/unread status, folders, and flags all stay in sync. The standard port is 993 (SSL). IMAP is the modern standard; see also: POP3.
Inbox Placement Rate Deliverability
The percentage of sent emails that land in the primary inbox (not spam, promotions, or other tabs). This is a more meaningful metric than delivery rate, which only measures whether the email was accepted by the server. Industry average inbox placement is roughly 85%.
Inbox Zero General
An email management philosophy coined by Merlin Mann in 2006, where the goal is an empty inbox at the end of each processing session. The principle is about deciding on each email quickly (delete, do, delegate, or defer) rather than literally having zero emails. See our guide: How to Achieve Inbox Zero with AI.
IP Reputation Deliverability
A score assigned to an IP address based on its email sending history. Shared by all senders using that IP (on shared infrastructure) or unique to you (on a dedicated IP). Poor IP reputation leads to emails being blocked or sent to spam.
IP Warm-Up Deliverability
The process of gradually increasing email volume from a new IP address to build sender reputation. Starting with small volumes to engaged recipients and slowly scaling up over 2–4 weeks. Sending too much too fast from a cold IP triggers spam filters.

J

Junk Mail General
Another term for spam. In Microsoft Outlook, the spam folder is called the “Junk Email” folder. See also: Spam.

K

Key Pair Security
A set of two cryptographic keys (public and private) used in DKIM and email encryption. The private key signs outgoing emails; the public key, published in DNS, lets recipients verify the signature. The keys are mathematically linked so only the correct private key produces a valid signature.

L

Landing Page Marketing
A standalone web page designed for a specific marketing campaign, often linked from an email. Landing pages focus on a single call-to-action (signup, purchase, download) and strip away navigation distractions.
List Hygiene Marketing
The practice of regularly cleaning your email list by removing invalid addresses, hard bounces, unsubscribes, and inactive subscribers. Good list hygiene keeps bounce rates low and sender reputation high. Verify addresses with our email verifier.
List-Unsubscribe Header Protocol
An email header (RFC 8058) that provides a one-click unsubscribe mechanism. Gmail and Yahoo require this header for bulk senders since 2024. It adds an “Unsubscribe” link in the email client interface, separate from any unsubscribe link in the email body.

M

Mail Server Protocol
A computer that handles sending, receiving, and storing email. Outgoing mail servers use SMTP; incoming mail servers use IMAP or POP3. Examples include Postfix, Microsoft Exchange, and Google’s Gmail infrastructure.
Mail Transfer Agent (MTA) Protocol
Software that routes email between servers using SMTP. MTAs are responsible for the server-to-server delivery of email. Common MTAs include Postfix, Sendmail, and Microsoft Exchange.
Mailto Link General
An HTML link that opens the user’s default email client with a pre-filled recipient address. Can also pre-fill subject, CC, BCC, and body. Format: mailto:[email protected]?subject=Hello. Create one with our mailto link generator.
MIME (Multipurpose Internet Mail Extensions) Protocol
A standard that extends email beyond plain text to support attachments, HTML content, images, and non-ASCII characters. Every modern email uses MIME encoding. The Content-Type header specifies whether an email is plain text, HTML, or multipart.
MX Record (Mail Exchange Record) Protocol
A DNS record that specifies which mail server handles email for a domain. When someone sends email to @yourdomain.com, their server looks up your MX records to find where to deliver it. Multiple MX records with different priorities provide failover. Look up yours with our MX lookup tool.

N

Newsletter Marketing
A recurring email sent to subscribers with updates, content, or promotions. Newsletters are the most common form of email marketing. Effective newsletters have consistent schedules, valuable content, and clear unsubscribe options.
No-Reply Address General
An email address (typically [email protected]) that doesn’t accept incoming messages. Used for automated notifications and transactional emails. Considered bad practice for marketing emails because it prevents recipients from responding and can trigger spam filters.

O

OAuth Security
An authentication standard that lets applications access your email account without seeing your password. When you “Sign in with Google” to connect an email tool, you’re using OAuth. It provides scoped, revocable access tokens instead of sharing credentials directly.
Open Rate Marketing
The percentage of delivered emails that were opened. Calculated as (unique opens / delivered emails) × 100. Apple’s Mail Privacy Protection (launched 2021) inflates open rates by pre-loading tracking pixels, making this metric less reliable than click-through rate.
Opt-In Marketing
Explicit consent from a recipient to receive marketing emails. Required by law in most jurisdictions (GDPR, CAN-SPAM, CASL). See also: Double Opt-In.

P

Phishing Security
A social engineering attack where fraudulent emails impersonate legitimate senders to steal credentials, financial information, or install malware. Phishing is the #1 vector for cyberattacks. Email authentication (SPF/DKIM/DMARC) helps prevent domain-based phishing.
Plain Text Email General
An email containing only unformatted text with no HTML, images, or rich formatting. Plain text emails have higher deliverability rates and are sometimes preferred for personal or sales outreach because they feel more authentic. Convert HTML to plain text with our converter.
POP3 (Post Office Protocol 3) Protocol
An older protocol for retrieving email from a server. POP3 downloads emails to a single device and typically deletes them from the server. It doesn’t sync across devices. Standard port is 995 (SSL). Largely replaced by IMAP for modern use. See also: IMAP.
Postmaster Deliverability
The administrator responsible for a domain’s email infrastructure. Also refers to tools like Google Postmaster Tools, which provide data on domain reputation, spam rates, and authentication results for emails sent to Gmail.
PTR Record (Pointer Record) Protocol
A DNS record that maps an IP address to a domain name (the reverse of an A record). Used in reverse DNS lookups. Many mail servers reject emails from IPs without valid PTR records. Check yours with our PTR checker.

Q

Quarantine Deliverability
In DMARC, a policy that instructs receiving servers to send unauthenticated emails to the spam/junk folder rather than rejecting them outright. Quarantine is the middle-ground DMARC policy between none (monitor) and reject (block).

R

Rate Limiting Deliverability
When an email server limits how many messages it accepts from a sender in a given time period. Exceeding rate limits triggers temporary rejections (soft bounces). Common when sending to Gmail, Outlook, or Yahoo in bulk.
Read Receipt General
A notification sent back to the sender when a recipient opens their email. Most email clients let recipients decline read receipts. Some tools (like Superhuman) use invisible tracking pixels instead, which work without recipient consent but can be blocked by privacy features.
Reply-To General
An optional email header that specifies a different address for replies. If set, clicking “Reply” sends to the Reply-To address instead of the From address. Commonly used when the sending address is a no-reply or system address.
Return-Path Protocol
The email address that receives bounce notifications. Set during the SMTP transaction, it may differ from the visible From address. SPF validation checks the Return-Path domain. See also: Envelope Sender.
Reverse DNS (rDNS) Protocol
Looking up the domain name associated with an IP address (the opposite of a standard DNS lookup). Mail servers use reverse DNS to verify that a sending IP has a valid PTR record matching its domain. Failed rDNS checks can cause email rejection.

S

S/MIME (Secure/Multipurpose Internet Mail Extensions) Security
A standard for public-key encryption and signing of email using digital certificates. S/MIME provides end-to-end encryption and sender verification. Requires certificate management, making it more common in enterprise environments.
Segmentation Marketing
Dividing an email list into groups based on shared characteristics (industry, behavior, engagement level, location). Segmented campaigns generate 14% higher open rates and 100% higher click rates than non-segmented campaigns (Source: Mailchimp).
Sender Reputation Deliverability
A score assigned to your IP address and domain by email providers based on sending behavior. Factors include bounce rates, spam complaints, authentication, engagement, and volume consistency. A good sender reputation is the single most important factor in deliverability.
Shared IP Deliverability
An IP address used by multiple senders (typically via an ESP). Your deliverability is partially affected by other senders sharing the same IP. Shared IPs are standard for low-volume senders; high-volume senders should consider a dedicated IP.
SMTP (Simple Mail Transfer Protocol) Protocol
The standard protocol for sending email between servers. SMTP handles the “push” side of email delivery — transferring messages from sender to recipient server. Standard ports are 25 (server-to-server), 587 (submission with STARTTLS), and 465 (submission with SSL). See also: IMAP, POP3.
Snooze General
An email client feature that temporarily hides an email and resurfaces it at a chosen time. Popularized by Google Inbox (discontinued 2019) and adopted by Gmail, Outlook, Superhuman, and others. Useful for deferring non-urgent emails.
Soft Bounce Deliverability
A temporary email delivery failure caused by issues like a full mailbox, server downtime, or message size limits. The sending server typically retries delivery for 24–72 hours. Persistent soft bounces should be treated as hard bounces. See also: Hard Bounce.
Spam General
Unsolicited bulk email, typically commercial. Approximately 45% of all email sent globally is spam (Source: Statista 2025). Spam filters use content analysis, sender reputation, authentication, and machine learning to identify and filter spam.
Spam Filter Deliverability
Software that analyzes incoming emails and separates spam from legitimate messages. Modern spam filters use machine learning, reputation databases, content analysis, and authentication checks. Examples include Gmail’s filters, SpamAssassin, and Barracuda.
Spam Score Deliverability
A numerical score assigned to an email indicating how likely it is to be classified as spam. Higher scores mean higher spam likelihood. SpamAssassin uses a default threshold of 5.0. Check your email’s spam score with our deliverability tester.
Spam Trap Deliverability
An email address used by ISPs and anti-spam organizations to identify spammers. Pristine traps are addresses that were never used by a real person. Recycled traps are old addresses that were abandoned and repurposed. Sending to spam traps severely damages sender reputation.
SPF (Sender Policy Framework) Security
An email authentication protocol that lets domain owners specify which mail servers are authorized to send email on their behalf. SPF works by publishing a DNS TXT record listing approved IP addresses and mechanisms. Receiving servers check this record to verify the sender. Check yours with our SPF checker.
SPF Flattening Deliverability
The process of converting nested SPF includes and redirects into a flat list of IP addresses. SPF records have a 10 DNS lookup limit; exceeding it causes authentication failures. Flattening keeps you under the limit. Use our SPF flattening tool.
STARTTLS Security
A command that upgrades a plain-text SMTP connection to an encrypted TLS connection. Unlike implicit TLS (port 465), STARTTLS starts unencrypted on port 587 and then negotiates encryption. Most modern email servers support STARTTLS for opportunistic encryption.
Subdomain Deliverability
A prefix added to your main domain (e.g., mail.yourdomain.com or news.yourdomain.com). Many organizations use subdomains for marketing email to protect their primary domain’s reputation. Each subdomain can have its own SPF, DKIM, and DMARC records.

T

Throttling Deliverability
Intentionally limiting the rate at which you send emails to avoid triggering rate limits or spam filters at receiving servers. ESPs typically throttle automatically. Manual throttling is important during IP warm-up periods.
TLS (Transport Layer Security) Security
A cryptographic protocol that encrypts data in transit between email servers. TLS replaced the older SSL protocol. When both the sending and receiving servers support TLS, the email content is encrypted during transmission (but not at rest). See also: STARTTLS.
Tracking Pixel Marketing
A tiny, invisible image (usually 1×1 pixel) embedded in HTML emails that loads when the email is opened, notifying the sender. Used to measure open rates. Apple Mail Privacy Protection and some email clients block tracking pixels to protect recipient privacy.
Transactional Email Marketing
Automated emails triggered by a user action — password resets, purchase confirmations, shipping notifications, account alerts. Transactional emails have the highest open rates (up to 80%) because recipients expect them. They’re exempt from most marketing consent requirements.
TXT Record Protocol
A DNS record type that stores text data. TXT records are used for SPF policies, DKIM public keys, DMARC policies, and domain verification. They are the backbone of email authentication — all three major protocols publish their configuration via TXT records.

U

Unsubscribe Marketing
The process of removing yourself from an email mailing list. CAN-SPAM requires all marketing emails to include an unsubscribe mechanism, and senders must process requests within 10 business days. Google and Yahoo now require one-click unsubscribe via the List-Unsubscribe header.

V

Validation (Email) Deliverability
Verifying that an email address is correctly formatted, has a valid domain with MX records, and the mailbox exists. Email validation before sending reduces bounce rates and protects sender reputation. Validate addresses with our email verifier.

W

Warm-Up Deliverability
The process of gradually increasing email sending volume from a new IP or domain to establish sender reputation. A typical warm-up starts with 50–100 emails per day to your most engaged subscribers and doubles every few days over 2–4 weeks. See also: IP Warm-Up.
Webhook Protocol
An HTTP callback that notifies your application when an email event occurs — delivery, bounce, open, click, or spam complaint. ESPs provide webhooks so you can react to events in real time. Gmail and Outlook use webhooks (push notifications) to notify apps of new emails.
Webmail General
Email accessed through a web browser rather than a desktop or mobile application. Gmail, Outlook.com, and Yahoo Mail are webmail clients. Webmail requires no software installation and is accessible from any device with a browser.
Whitelisting (Allowlisting) Deliverability
Adding a sender’s email address or domain to an approved list so their emails bypass spam filters. Asking recipients to whitelist your address is a common deliverability tactic. The industry is shifting to the term “allowlisting” for inclusive language.

X

X-Headers Protocol
Custom email headers prefixed with “X-” that carry non-standard metadata. Examples include X-Mailer (the sending application), X-Spam-Score (spam filter results), and X-Priority (message urgency). While the “X-” convention is technically deprecated by RFC 6648, it remains widely used.

Z

Zero-Access Architecture Security
A security model where the service provider cannot access user data, even if their servers are compromised. Data is encrypted with user-specific keys that the provider doesn’t hold. Unboxd uses zero-access architecture with AES-256-GCM encryption and per-user keys derived via PBKDF2.

Frequently asked questions

What is SPF in email?

SPF (Sender Policy Framework) is an email authentication protocol that lets domain owners specify which mail servers are authorized to send email on their behalf. It works by publishing a DNS TXT record listing approved IP addresses. Receiving servers check this record to verify the sender is legitimate, helping prevent email spoofing and phishing.

What is the difference between IMAP and POP3?

IMAP (Internet Message Access Protocol) keeps emails on the server and syncs across all your devices, so changes on one device appear everywhere. POP3 (Post Office Protocol 3) downloads emails to a single device and typically deletes them from the server. IMAP is the modern standard for anyone using email on multiple devices; POP3 is only used in niche cases where offline-only access is required.

What does DMARC do?

DMARC (Domain-based Message Authentication, Reporting, and Conformance) is an email authentication policy that tells receiving mail servers what to do when an email fails SPF or DKIM checks. It can instruct servers to deliver, quarantine, or reject unauthenticated messages. DMARC also provides reporting so domain owners can monitor who is sending email using their domain.

What is the difference between CC and BCC in email?

CC (Carbon Copy) sends a copy of the email to additional recipients, and all recipients can see who was CC’d. BCC (Blind Carbon Copy) also sends a copy to additional recipients, but their addresses are hidden from all other recipients. Use CC when transparency matters; use BCC for mass emails or when you want to protect recipients’ privacy.