Gmail enforces bulk-sender rules: authentication, one-click unsubscribe, and spam-rate thresholds under 0.3% in Postmaster Tools.
What email subject lines are#
The subject line and preheader are the two text fields shown in the inbox list before a message is opened. They determine whether a recipient opens, ignores, or reports the message.
Why it matters#
Subject lines are the highest-leverage copy you write. They also affect complaint rates, which feed reputation.
Setting up email subject lines for Gmail#
- Keep subject lines under 50 characters so they survive mobile truncation.
- Write the preheader as a continuation, not a repeat of the subject.
- A/B test one variable at a time with a statistically meaningful sample.
- Avoid deceptive 'Re:' or 'Fwd:' prefixes, which violate CAN-SPAM and raise complaints.
How Gmail reports results#
Open a delivered test message in Gmail and view the original or headers. Look for Authentication-Results and any provider-specific verdict headers. Use the provider's sender dashboard for aggregate reputation.
Common mistakes#
- Clickbait that produces opens but drives complaints and unsubscribes.
- All-caps and excessive punctuation, which pattern-match to spam.
Frequently asked questions#
Do emojis in subject lines hurt deliverability?
Not directly. They can hurt engagement with B2B audiences and render inconsistently across clients.
Does Gmail require email subject lines?
Gmail enforces bulk-sender rules: authentication, one-click unsubscribe, and spam-rate thresholds under 0.3% in Postmaster Tools. Treat email subject lines as required for any meaningful volume.