Moving an audience between platforms is the single riskiest thing an email operator does. It is also usually done in an afternoon, under time pressure, with a CSV.
Here is the pre-flight list, ordered by how expensive the mistake is to undo.
1. Can you export the suppression list?
Do this before anything else, because it is the item most likely to be missing and the one whose absence causes the most damage. If you import contacts without importing the addresses that unsubscribed, bounced, or complained, your first send on the new platform goes to every one of them.
That is not a slow-burn problem. It is a complaint spike on a cold domain, which is the worst possible first impression to make on a mailbox provider.
2. What consent evidence are you actually carrying?
There is a difference between “these people are marked subscribed in the old system” and “I have a record of when and how each of them consented”. Only the second survives a challenge.
Decide honestly which you have, then import accordingly — preserving genuine evidence, requiring re-confirmation where you have none, or recording an explicit attestation with your name attached. What you must not do is import with a blanket subscribed flag and no provenance. That converts an unknown into a false claim.
3. Are you re-warming, or inheriting?
If you are also changing sending infrastructure — new provider, new IPs, new domain — your reputation does not travel with the contacts. Starting at your previous full volume on cold infrastructure is the classic way to get filtered.
Plan a warmup: daily limits with a growth target, ramping over weeks rather than days, and lead with your most engaged segment. Your best-performing contacts are the ones that teach mailbox providers you are worth delivering.
4. Have you authenticated the new domains?
SPF, DKIM, and DMARC alignment need to be right before the first send, not diagnosed after it. Verify every domain by purpose — sending, tracking, return-path — and confirm the from-address on your sender profile actually aligns with a verified sending domain.
A single misaligned return-path can quietly halve delivery, and it is nearly invisible unless you are looking at bounce categorisation.
5. What does your data model actually look like?
CSV columns are all strings. Your new platform’s fields probably are not. Decide the type for every custom field before import — text, number, boolean, date, JSON — because a date column that lands as free text will silently break every segment that depends on it, and you will not notice until a birthday campaign sends to nobody.
While you are there, mark the sensitive fields. It costs nothing at import and it is what lets retention and privacy tooling treat them correctly later.
6. Are duplicates a merge, a skip, or a reject?
Multi-source audiences always contain duplicates. Decide up front whether a matching contact gets updated, skipped, or rejected. The dangerous default is a silent merge, because it can overwrite good data with stale data from a secondary export and leave no record that it happened.
7. Can you get back out?
Before you commit, confirm the new platform can export contacts, consent records, suppressions, and form submissions in a usable format. A platform that is easy to enter and hard to leave has told you something about how it expects to retain you.
The order to actually do it in
- Export everything from the old system, including suppressions and consent history.
- Set up the new brand, legal entity, and domains. Verify them.
- Import suppressions first. Always.
- Import contacts with an explicit consent strategy.
- Send to your most engaged segment only, at low volume.
- Watch bounce and complaint rates for a week before scaling.
- Keep the old platform live and read-only for a month.
That last one is not paranoia. The failure you did not anticipate always shows up in week three, and the ability to compare against the source of truth is worth one more month of subscription.