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.
How mailbox providers use it#
Gmail, Microsoft, and Yahoo combine authentication results, sender reputation, and recipient engagement into a placement decision made per message. Email subject lines feeds directly into that model, and weaknesses compound with other signals.
How to measure the impact#
- Baseline inbox placement with seed tests before any change.
- Make one change at a time and hold volume steady.
- Re-test after 48 to 72 hours; provider models need time to update.
- Track Postmaster Tools and SNDS alongside your seed results.
Improving it#
- 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.
Frequently asked questions#
Do emojis in subject lines hurt deliverability?
Not directly. They can hurt engagement with B2B audiences and render inconsistently across clients.