How to write effective email subject lines: tips and best practices
What is the ideal length for an email subject line?
The sweet spot is 6-10 words or 30-60 characters. Subject lines in this range display fully on both desktop and mobile clients, giving recipients enough context to decide whether to open. Shorter lines may lack context, while longer ones risk being truncated -- especially on mobile, where most emails are read today.
Which words trigger spam filters?
Common spam trigger words include "free," "act now," "limited time," "winner," "congratulations," "click here," and "guaranteed." Excessive use of capital letters, exclamation marks, or symbols like "$$$" also raise red flags. While modern spam filters use AI-based analysis beyond simple keyword matching, avoiding these patterns reduces the risk of landing in spam folders.
How do I improve my email open rates?
Write subject lines that are specific and relevant to the recipient. Create curiosity without resorting to clickbait. Personalize when possible -- emails with personalized subject lines are 26% more likely to be opened. Use numbers, ask questions, and create a sense of timeliness without fake urgency. Most importantly, deliver on the promise your subject line makes.
Should I A/B test my email subject lines?
Absolutely. A/B testing (sending two versions of a subject line to small segments before sending the winner to the full list) is one of the most reliable ways to improve open rates over time. Test one variable at a time: length, tone, personalization, or the use of numbers vs. text. Even small improvements in open rate compound significantly across campaigns.
Does personalization in subject lines really work?
Yes, but it needs to be done well. Using a recipient's first name can boost open rates, but generic personalization (like company name) has diminishing returns as it becomes more common. The most effective personalization references something specific -- a recent interaction, a shared interest, or a relevant pain point. The key is making the recipient feel the email was written specifically for them.
