AI email subject line generator -
five lines that actually get opened
Paste the body, pick a goal, get 5 subject lines built around what you wrote. No more "Following up." No more "Quick question." Just lines that earn the open.
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Create free account →Most people decide to open on the subject alone
The average inbox takes over 100 emails a day. Yours gets about 3 seconds and 60 characters to earn attention before it scrolls past. A better subject line isn't a minor tweak — it's the line between read and ignored.
Replai reads your email body for context, intent, and the real ask, then writes subject lines that fit your goal — prompt a reply, signal urgency, or say plainly what's inside.
Reply rate goal
Urgency goal
Clarity goal
One size does not fit all
Cold outreach, internal updates, client proposals, and follow-ups each call for a different strategy. A cold pitch needs curiosity without clickbait — numbers and specifics ("3 ideas for your Q3 pipeline") beat vague openers ("Quick question," "Reaching out") every time. An update to a senior stakeholder needs clarity first: they should know what it's about before they open it, not after.
Follow-ups are where subject lines fail most. "Following up on my last email" reads as low priority and gives no new reason to act. Replai writes lines that nod to the prior email without repeating it and frame the ask so replying feels easy. Need the follow-up body too? Use the Follow-up Generator. And before any email goes out, the Email Tone Checker catches passive-aggressive phrasing or vague asks that cost you the reply.
The goal setting tells the AI what you actually want — not just "open it," but reply, act, or understand. That intent shapes every line. The same body can produce wildly different subject lines depending on whether urgency or clarity leads. For the bigger picture on AI in your inbox, see the best AI email assistants compared.
Subject line patterns by use case
Cold outreach subject lines
Specific beats clever. "3 ideas for {company}'s onboarding emails" beats "Quick question" because it proves you did the homework before asking for attention. Keep it under 50 characters so nothing truncates on mobile, name the value not the meeting, and never fake a reply with "Re:" — it wins the open and loses the trust.
Internal and team update subject lines
Lead with the category and the action: "Q3 budget - approval needed by Friday" tells a busy stakeholder what it is, what you need, and when — before they open it. Flag FYI-only emails ("Launch metrics - no action needed") and your action-required ones get opened faster, because the label actually means something.
Follow-up subject lines
Give a new reason to act, don't restate the old one. "Updated pricing for the option you preferred" reopens a stalled thread; "Following up again" buries it. Reference what changed since last time — a deadline, a new detail, an easier ask — and keep the original thread subject only when the recipient needs it to place you.