Migrating your list

Moving an existing audience into Mimeo without torching the sender reputation you're arriving with. Simple, boring, and worth doing in order.

Export everything, including the opt-outs

From your old tool, export your full list and your unsubscribes and suppressions. The opt-outs are the most important file in the migration: importing a list without them means re-mailing people who already said no — the fastest known route to spam complaints.

In Mimeo's CSV import, map the relevant column to already unsubscribed (or run the opt-out file as its own import with every row marked unsubscribed). Unsubscribed people stay on the books precisely so a later import can't accidentally revive them.

Bring the subscribe dates with you

Export the column holding the date each person joined, and map it to subscribed on during import. Every person in Mimeo has a subscribed-on date — people who join through Mimeo get the day they arrived — so once your list is in, "who joined before the cutover?" is a single rule and not a special case you have to remember forever.

The date only ever moves earlier. An import carrying a date older than the one Mimeo has corrects the record; one carrying a newer date is ignored. That's what makes it safe to re-run an import, to import your list in batches in any order, and to have your site keep sending its opt-in events afterwards without the repeat opt-ins slowly rewriting everyone's history.

If you want to correct one person by hand, edit the date on their profile — that sets it outright, in either direction.

Backfilling history won't send mail

The dangerous-looking part of any migration is loading the past: everyone's opt-in, everyone's purchase history. Recording those as events would normally wake the automation engine, and replaying 19,000 opt-ins would start 19,000 journeys and mail every one of them.

Mark them historical and that can't happen. The event is recorded in full — timeline, money tables, segment conditions — but it starts no flow and releases nobody waiting at a gate. CSV imports do it for you automatically, including for the tag_added events an import emits for every tag it applies.

Every flow ignores historical events unless you deliberately tell it not to, and the checkbox on a flow's entry rules shows you which way it's set. You do not need to deactivate your flows before a backfill — which matters most later, when you want to load old data into an instance that's already running and can't be taken down.

Trim before you import

Only bring people who actually opted in and could reasonably remember you. Leave behind hard bounces, role addresses (info@, sales@), and anyone who hasn't opened or clicked in a year or more — mailing years-cold addresses from a new setup produces bounces and complaints exactly when your reputation can least afford them. A smaller, warmer list delivers better than a bigger, colder one, every time.

Warm up before you blast

Inbox providers judge mail partly by the reputation of the sending domain and IP behind it — and your provider account, your domain's sending pattern, or both are new. Reputation is earned by sending modest volume that people demonstrably want, then growing it.

Start with your most engaged segment — recent openers, recent customers — and a send they'll genuinely welcome. Then roughly double the audience every few days while the signals stay clean. A sensible shape for a 20,000-person list:

Segments make each rung a rule rather than a spreadsheet. If you imported engagement data as fields or tags, segment on that; if not, start with customers and known-active people and let Mimeo's own open data take over from there.

Watch reporting after each rung. Bounces under 2% and complaints under 0.1% mean climb on; numbers above that mean pause, trim deeper, and let a few quiet days pass before the next rung. One more provider-side note: Postmark approves accounts for full sending after seeing clean traffic, so the early rungs also serve as that audition.

Cut over in one direction

Run the two tools in parallel only as long as the migration takes, and let each own what it owns: once a flow or sequence is rebuilt in Mimeo, turn it off in the old tool the same day, so nobody gets both versions. Point your signup forms, checkout and app at Mimeo's events API early — ongoing data is worth more than any snapshot. Finish with a final delta export from the old tool (new signups and new unsubscribes since your first export), import it, and close the account.

What Mimeo handles for you

Suppression is enforced at send time by a built-in guard, so an unsubscribed, bounced or complaining person can't be mailed even by a mis-built segment. Every marketing email carries the one-click unsubscribe header Gmail and Yahoo require. And the scheduled queue shows every pending send with a reason, so "what's about to go out?" is always answerable — the question that matters most in migration week.