Deliverability Dropped After Switching ESP or Replatforming? A Recovery Order

When deliverability drops after an ESP move or a commerce replatform, the usual cause is that receivers now see a new sender: new IPs, often a new DKIM or return-path domain, and a different mix of mail, sent at full volume too soon. Re-authenticating fixes the "is this allowed" question, not the "do we trust this" one. Make sure suppressions came across, split transactional from marketing mail, ramp only your most engaged contacts first, and change one thing at a time.

A thread in r/Emailmarketing describes the pattern: a couple of months after a replatform, inbox rates were down at the main providers, more mail was in Gmail's Promotions tab, and Microsoft SNDS showed more red, despite DKIM, SPF and DMARC redone against the new IPs, a two-week graduated warm-up, a hard-bounce scrub and a re-engagement push. Here is the order we'd work through.

1. Find what actually changed

Compare an old message and a new one with the email header analyzer, and note:

Check	Why it matters
Sending IPs, and whether they're shared or dedicated	New IPs have no history with Gmail or Microsoft
DKIM d= domain	If it moved from your domain to the ESP's, or to a new subdomain, domain reputation starts over
Return-path (envelope from) domain	Affects SPF alignment and which domain bounces are attributed to
From address and display name	Recipients recognise these, and Google asks for one From address per message type
Which streams share an IP	Order confirmations and promotions on the same new IP share one reputation
List-Unsubscribe headers	Gmail and Yahoo require one-click unsubscribe for bulk marketing mail

2. Check that suppressions migrated

Replatforms often import "all customers" from the commerce backend instead of the ESP's subscriber list. That brings back people who unsubscribed, hard-bounced or complained on the old platform. Export the old platform's suppressed, bounced and unsubscribed contacts and re-apply them. Our suppression list merger merges several exports into one file, and our post on switching platforms covers what to export.

3. Split the streams

Google's sender guidelines recommend separate From addresses per category of mail, and, if you use more than one IP, a separate IP per message type. They also say not to mix content types, "such as promotions in sales receipts." Put transactional mail on its own stream first. It's engaged mail and it rebuilds reputation fastest.

4. Ramp again, slower and narrower

Google: "Start with a low sending volume to engaged users, and slowly increase the volume over time," and after significant infrastructure changes, ramp the changed traffic separately. In practice:

1. Send marketing only to people who opened or clicked in the last 30 to 60 days.
2. Hold the dormant segment back entirely until the engaged sends look healthy.
3. Grow volume in steps and hold each step until the metrics stay flat.

The email warmup calculator gives a step schedule. If you already re-engaged the dormant segment in the first weeks, that may be what's holding reputation down. Our sunset policy guide covers how to retire it.

5. Clean before every widening step

Before you add an older segment, verify it on the verify page. Addresses that went bad while you weren't mailing them are a bounce spike on a reputation that's still forming.

6. Measure with the providers' own data

Use Google Postmaster Tools for spam rate and domain reputation, and Microsoft SNDS for your new IPs. Keep Gmail spam rate below 0.1% and never at 0.3%, as Google's guidelines say. Log every change with a date, and change one thing per week so you can see what helped.

What SimpleVerifier costs

The first 100 checks are free with a Google sign-in. Pro is $39 a month with the first 100,000 checks a month at full speed, then slower, never cut off, and one-time packs of 10,000 checks for $15 never expire. See pricing.