Switching Email Platforms? Clean and Verify the List First

Verify your list before you import it into a new email platform, not after the first campaign bounces. A migration combines three risks at once: a sending setup with no history, a new provider with its own bounce and complaint limits, and a list that has usually not been cleaned since the last platform was chosen. Export suppressions, verify, segment by engagement, then ramp up.

This guide covers that order of operations, with each platform rule taken from the provider's own documentation as read in October 2026.

Why a migration is the riskiest send you will make

On a new platform you have no reputation to absorb mistakes, and the platform is watching newly imported contacts closely. On the old ESP, a slightly dirty list was diluted by years of normal sends. On the new one, your first campaign is most of what the platform and the mailbox providers know about you.

Several providers say this outright:

• Mailchimp runs large imports through its abuse-prevention system, Omnivore. Its help page says "the first email campaign you send to this newly updated audience will send slowly so that our system has time to verify the imported addresses", and if that check fails, "we'll cancel the remaining sends to the newly imported contacts and we may ask that you undo your import."
• Brevo says a campaign "sent to new contacts" is one of the triggers for review. It assesses a random subset first, and its suspension page lists a hard bounce rate above 2%, an unsubscribe rate above 1% or a complaint rate above 0.2% as grounds for suspending campaigns or the account.
• Amazon SES places every new account in a sandbox limited to verified recipients, 200 messages per 24 hours and one per second. Once out of it, SES reviews accounts at a 5% bounce rate or 0.1% complaint rate.
• Klaviyo says in its domain migration guide that new accounts, and domains registered for less than 30 days, "should plan to warm their sending infrastructure".

The full table of published limits is in bounce rate limits by platform. The point for a migration is that the lowest of them, Brevo's 2%, is easy to break with a list that has been sitting untouched for a year.

What you can and cannot bring with you

Your list, your consent records and your suppressions move with you; your old sending reputation may not. Whether any reputation carries over depends on what changes:

What changes in the move	What that means
Sending IP	If you move to the new ESP's shared pool or a new dedicated IP, any IP reputation you had stays behind. A new dedicated IP needs warming. See shared vs dedicated IP
Sending domain stays the same	Domain reputation travels with the domain, which helps. You still need new SPF and DKIM records for the new platform
New sending domain or subdomain	No history at all. Treat it like a brand new domain
List	Comes with you, including every dead address the old ESP was quietly suppressing

That last row is the one that catches people. Your old platform marked bounced addresses as "cleaned" or "suppressed" and stopped sending to them. A raw export of "all contacts" can include them again, and the new platform has no record that they ever bounced.

Step 1: export suppressions before anything else

Export unsubscribes, hard bounces and spam complaints as separate files, and load them into the new ESP's suppression list before your first import of subscribers. This is the step people skip, and it is the one with legal consequences: emailing someone who unsubscribed breaks the opt-out rules in CAN-SPAM and its equivalents, whichever platform you use.

How the main platforms handle this:

• Klaviyo's migration guide says that with a built-in integration "you're all set", but otherwise "you will need to ensure any lists of bounces/unsubscribes are uploaded directly to your suppression list". Its domain migration guide adds that you should bring "subscription method and timestamp for email and SMS opt-ins, along with all unsubscribe and bounce data".
• Mailchimp lets you import a suppression list by choosing the email marketing status Unsubscribed during import.

Keep the consent data too: where each subscriber signed up and when. If the new platform ever asks how a contact opted in, that is your answer.

Step 2: remove what you are not allowed to send to

Before verification, take out anything without consent, because no verifier can make a purchased list compliant. The policies are blunt:

• Mailchimp's Acceptable Use Policy prohibits sending to "purchased, rented, third-party, co-reg, publicly available data, or partner lists of any kind", and says you must be able to point to an opt-in form or other evidence of consent.
• Klaviyo's Acceptable Use Policy prohibits contacts that are "purchased, rented, borrowed, third-party owned/provided" and also bans "waterfalling or list cleaning across multiple email or mobile messaging service providers", meaning you cannot use one platform to burn through a bad list and then move the survivors.
• Amazon SES's spam trap guidance says "sending to purchased or rented lists is prohibited by the Amazon SES terms of service".

If part of your list came from a source you cannot account for, a migration is a good moment to leave it behind. Our post on buying email lists explains why those addresses cause the most damage.

Step 3: verify the remainder

Run every contact you intend to import through an email verifier, and import only what comes back valid, with risky held in a separate group. This removes the addresses that would hard bounce on your first campaign, which is exactly the metric the new platform is watching.

What to do with each result:

Result	Import?
Valid	Yes
Invalid	No. Add to the suppression file from Step 1 so it is never re-imported
Risky (catch-all, or the server would not answer)	Import into a separate segment and leave it out of the first sends
Disposable	No

Two practical notes. First, verify close to the import date, not weeks before, because a valid result describes the mailbox on the day it was checked. Second, if your list is large, split the file before you start: SimpleVerifier, for example, accepts CSVs of up to 100,000 rows per upload and costs $29.99 a month with no per-address charge, so a list split into several files costs the same as one. Any verifier will do, as long as it reports catch-all addresses as risky rather than valid.

Step 4: segment by engagement before you import

Bring engagement data from the old platform, because the new one cannot tell your active readers from your silent ones. Klaviyo's migration guide recommends that you "import clean lists into Klaviyo and send to an engaged list from your first send", and suggests using your old ESP's reporting to isolate inactive contacts first.

A simple split:

1. Engaged: clicked, replied or bought in the last 90 days.
2. Lapsing: engaged in the last 90 to 180 days.
3. Inactive: nothing in 180 days or more.

Use clicks and purchases rather than opens, because Apple's Mail Privacy Protection records opens nobody made. The inactive group should not be part of the migration sends at all. Run it through a re-engagement campaign once the new setup is stable, as described in the sunset policy guide.

Step 5: ramp up from the engaged segment

Your first sends on the new platform go to your best contacts only, at a modest volume, and grow from there. Gmail's sender guidelines say to "start with a low sending volume to engaged users, and slowly increase the volume over time", and to avoid sending in bursts. Klaviyo's guide suggests sending to the engaged segment for the first week if you send daily, or for your first two or three campaigns if you send more than once a week.

The exact pace depends on your volume and whether the IP is shared or dedicated. A day-by-day plan is in our email warm-up schedule.

Before the first send, check that the new platform's SPF and DKIM records are live for your domain and that DMARC still passes. The DKIM checker shows whether the new selector is published and readable.

Migration checklist

1. Export unsubscribes, bounces and complaints separately. Load them into the new suppression list first.
2. Export consent source and timestamp with each subscriber.
3. Remove contacts with no evidence of consent.
4. Verify the rest. Import valid; hold risky separately; suppress invalid.
5. Split by engagement using clicks and purchases, not opens.
6. Publish SPF and DKIM for the new platform and confirm DMARC passes.
7. Send to the engaged segment first, at low volume, and increase gradually.
8. Re-engage or retire the inactive segment once sending is stable.

The old platform was doing list hygiene for you in the background. On the new one, nothing has been cleaned until you clean it.