A lifecycle email's subject line is the one variable you usually get no real signal on from a single send. Add up to 4 more subject variants to the main one, and each recipient is deterministically assigned a variant based on their email address — stable across retries — so open rate and response rate show up side by side once the send goes out.
A trial-expiry or renewal email only gets one real shot per recipient — testing the subject line on that one shot, rather than guessing, is worth the extra thirty seconds of setup.
Test up to 5 subject lines at once
Add up to 4 more variants under the main subject field on a one-time send — 2 to 5 total. Each recipient's assignment is a deterministic hash of their email address, so a retried or resent message never reshuffles who saw which line.
Read the results side by side
The Responses page adds a variant comparison once a multi-subject send goes out — open rate and response rate per variant, with the current leader flagged, cumulative across every A/B send the project has ever run rather than scoped to just the latest one.
Carry the winner into your ongoing cadence
Recurring sends stay single-subject — there's no variant picker on the schedule form. Once a variant wins on a one-time test, set it as the single subject line on the recurring schedule that reaches this lifecycle stage going forward.
It rides along for free downstream
The assigned variant index lives in the recipient's own live-view token, so every interaction event and project webhook payload for that recipient already carries it — no extra joins needed to see whether a winning subject line also produced more clicks or purchases, not just opens.
Getting started
On a one-time lifecycle send (a trial-expiry or renewal reminder), add 2-5 subject-line variants and send it. Check the variant comparison on the Responses dashboard, then set the winning subject as the single line on your recurring schedule for that same lifecycle stage.