Subject line
The email version of a headline, scanned beside the sender name before someone decides whether the message is worth opening.
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What it is
A subject line is the email's inbox headline. Alongside the sender name and timing, it tells a recipient what the message is and whether opening it is worth the interruption. Strong subject lines name the specific value, event, or required action in words the recipient can recognize at a glance.
Write it after the email has one clear job. Front-load the distinctive words, keep the promise consistent with the body, pair it with preheader text that adds information, and test meaning before cleverness. For transactional mail, clarity such as 'Your receipt for order 4821' beats suspense.
Gotcha: open rate is a noisy judge. Apple Mail Privacy Protection, offered to every user since iOS 15 in 2021, can load tracking pixels without a person reading the message, so a subject-line test should also watch clicks, replies, conversions, or completed actions. Curiosity that wins an open and breaks the email's promise teaches recipients not to trust the next one.
Ask AI for it
Write 12 subject lines for this email and group them into four Mailchimp A/B test hypotheses: specific benefit, urgency, direct question, and plain description. Keep each under 50 characters, put the differentiating words first, and pair each with a preheader that adds rather than repeats information. For every option, state the promise it makes and the click or conversion event that should judge it. Reject any line the email body cannot prove.