A platform move resets several things at once. New sending IPs, often a new sending domain or subdomain, and new authentication records. To a receiving provider that combination looks like a new sender, and new senders are treated cautiously by default.
Suppression data is the part most often lost. Unsubscribes, complaints and bounce history live in the old platform, and exporting them is a manual step that gets skipped under deadline. Mailing someone who opted out two years ago is both a compliance problem and a complaint.
So the migration checklist has three items before the first send: export and import the suppression list, verify the contacts you are bringing, and segment by engagement so the first sends go to people who actually open.
Klaviyo's own deliverability guidance points the same way, recommending a gradual volume ramp to the most engaged recipients. That advice only works if the list underneath it is clean, because a bounce during a ramp is unusually expensive.
Do not bring everything. A migration is the natural moment to drop the segment that has ignored you for two years, since you are rebuilding the sender's reputation from scratch anyway.
An 80,000-profile move, week by week
Say you're moving 80,000 profiles. In the week before cutover, export three things from the old platform: unsubscribes, complaints and hard bounces. Then verify the full contact file. Suppose 9,000 come back invalid, and another 12,000 haven't opened anything in two years. You leave both groups behind and migrate 59,000.
Week one on the new platform goes to the 14,000 people who opened or clicked in the last 30 days. In week two you add the 60-day group, and in week three the 90-day group. The rest follow in week four, and only if bounce and complaint numbers stayed quiet.
Load the suppression files before the first send, not after it. A migration that mails even one person who unsubscribed back in 2023 has already earned its first complaint before the ramp begins.
What gets left behind in a platform move
Engagement history is the big one. A plain contact export carries names and addresses, and it usually leaves out the date each person last opened or clicked. Without that date, you can't build the engaged-first segments a careful ramp depends on. Export it as a column and import it as a custom property, so the segments work on day one.
Forms are next. Signup forms on your site and in old landing pages may still post to the old platform. List them all before cutover, because new signups that land in a closed account are lost quietly.
Check your sending domain records as well. The new platform will ask for its own DNS entries. Until those pass, your mail goes out under a shared domain with a reputation you don't control.
Keep billing in mind too. Klaviyo prices by active profiles, so contacts you suppress or leave behind stop costing you money.
Related questions
What should I do before any platform migration?
Export suppressions, verify contacts, and segment by engagement. The first two are compliance and reputation; the third protects the ramp.
Should I migrate my whole list?
No. A migration is the right moment to drop long-dormant subscribers, since you are rebuilding reputation regardless.
Do suppression lists transfer automatically?
Not usually. Export and import them yourself, before the first send on the new platform.
How fast can I ramp up after moving?
Gradually, over several weeks, starting with the most engaged recipients. Watch bounce and complaint rates between steps.