How to Migrate ESPs Without Losing Deliverability

The contract is signed, your flows are rebuilt, and the obvious move is mailing the full list on Monday so the new platform “starts clean.” It will not start clean. Gmail and Yahoo already have a history for your domain. The new platform has a history for its IPs. Monday’s job is to connect those two without a day of authentication failures and a day of complaints.

This is that sequence for your store. DNS record shapes live in SPF, DKIM, and DMARC and, for Klaviyo, in the authentication guide. This page is the order of operations, not a second copy of those records.

Last verified: 26 September 2026. Provider rules cited here are the ones documented in the Gmail, Yahoo, and Outlook 550 5.7.515 guides.

What moves, and what does not

Switching ESPs changes your IPs and leaves your domain’s reputation exactly where it was. That cuts both ways: a new shared pool can be better or worse than the old one, and nothing in your domain history is erased or granted. IP reputation and domain reputation are the two scores you are most likely to mix up during a migration. Leaving a shared IP pool can remove a neighbor’s problem. It does not erase a spam rate your domain already earned. Joining a new shared pool can add a neighbor’s problem you did not have.

If the plan also changes the domain customers see, that is a different project: changing the sending domain. Do not stack it on the platform cutover unless you have already warmed the new domain.

Before anyone is mailed

Inventory every sender. Campaigns, welcome, cart, browse, post-purchase, reviews, shipping, the helpdesk, and any app that sends as @yourstore.com. The one you forget is the one that fails DMARC on Friday.

Move suppressions first. Unsubscribes, spam complaints, and hard bounces. A new Klaviyo account or a new SES account will happily mail people the old account had stopped mailing. That is how cutover day produces the complaint spike.

Authenticate before the import finishes, not after the first blast. Gmail’s bulk rules want SPF and DKIM, plus DMARC, once you send close to 5,000 or more messages to personal Gmail accounts within a 24-hour period. Yahoo’s bulk rules want both SPF and DKIM and a DMARC pass. Microsoft returns 550 5.7.515 when a sender of more than 5,000 emails a day to its consumer mailboxes fails SPF, DKIM, or DMARC. Add the new platform’s include and key while the old platform’s records are still in place, or you will have a gap where neither is valid. SPF’s 10-lookup limit is the usual way a “just add the new include” breaks the old one. Check the lookup count before you publish.

Decide what stays transactional. Order and shipping mail are the messages that become support tickets when they vanish. If they move to SES, read the SES deliverability guide first. A pause on that account stops receipts, not only newsletters. If they stay on the old platform for a week, say so in the plan so nobody turns the old account off.

The cutover itself

1. Send the new platform only to people who clicked recently. This is a placement test with real customers, small on purpose.
2. Read Authentication-Results on a received copy. DNS looking correct in the host panel is not the test. A pass on the message is the test.
3. Check placement at Gmail, Yahoo, AOL, and Outlook. InboxEagle’s seed list covers those four. Outlook authentication rejections happen at SMTP time, so read the bounce log for 5.7.515.
4. Turn on one flow family, or one campaign type, not the entire lifecycle map. Flows versus campaigns still matters on the new account: two emails the same afternoon feel like a new sender even when the domain is old.
5. Watch Postmaster Tools spam rate and Yahoo complaints the next day. Gmail does not send those complaints to the ESP. A calm new-account dashboard is not a Gmail result.
6. Expand the audience in steps. If the engaged slice goes to spam, stop. Going back to the full list will not average the problem away.
7. Remove the old platform from SPF only after it has stopped sending. Leaving a dead include costs you lookups. Removing a live one causes failures.

Klaviyo’s IP and domain warm-up is the Klaviyo-specific version of step 6. Use it when Klaviyo is the destination. It does not replace the suppression import.

How to verify the migration worked

• Authentication passes on campaigns and on a real order message, if orders moved.
• Placement on the engaged slice matches the recent baseline closely enough that you can explain the gap. A new IP can sit lower at first. A collapse to spam is not “warm-up.”
• Bounce codes are not a new population of 5.7.515 or authentication failures.
• Spam rate at Gmail is not climbing toward 0.30%. Google’s FAQ says a bulk sender at or above that rate is ineligible for mitigation until seven consecutive days under it. You do not want to spend the first week of a new contract there.
• Unsubscribes from the old platform are suppressed on the new one. Sample ten known unsubscribes and confirm they were not mailed.

ESP migration mistakes to avoid

• Do not migrate to escape a spam-folder problem. Fix the audience and authentication on the current platform, using the first 24 hours, then move when the metrics are stable.
• Do not buy a list to “fill” the new account.
• Do not judge the new platform by its open rate in week one. Open rate is a weak signal, and a new sending setup changes image loading and filtering at the same time.

When to escalate an ESP migration problem

Escalate to the new ESP when the DNS records match their documentation and live mail still fails DKIM or SPF. The signature they applied and the key you published disagree.

Escalate internally, and pause the expansion, when the engaged slice lands in spam at Gmail and Yahoo. That is a reputation or list problem you imported. More volume will not clarify it. The ramp after a platform change follows the same stop rule as any other cold path: warmup failure if placement or bounces break while you are still adding people, and the warm start on changing a sending domain if the From domain is new as well as the platform.

What to monitor after the cutover

For the first campaigns and the first week of flows, watch placement by provider, authentication per send, Gmail spam rate, and Yahoo complaints. The Klaviyo integration is the path when Klaviyo is the new platform. The SES integration is the path when SES is. Neither one imports your suppressions for you.

Sources

• Email sender guidelines, Google. Authentication requirements that still apply on the new platform.
• Sender best practices, Yahoo. DMARC pass and complaint-rate rules that do not reset when you change ESPs.
• Fix NDR 550 5.7.515, Microsoft. Rejection when the From domain is not authenticated.
• Amazon SES sending review FAQ, AWS. Review and pause behavior if the destination is SES.