The short answer: in most cases the suppression list did not migrate. Unsubscribes, hard bounces, and spam complainants live in your old platform's suppression records, and plenty of migrations move profiles while quietly leaving suppressions behind. You resume mailing people who already told you to stop, complaints spike, and inbox placement falls for everyone on the list, including your best customers.
The symptom usually shows up two to three weeks after launch, well after the migration has been declared finished, which is why it often gets blamed on content or send frequency instead.
Why the suppression list is the usual culprit
Suppression records are the one part of your email data that has no equivalent in your product catalog or order history, so they are easy to forget in a migration plan built around products and customers.
Worse, the failure can happen even when someone did think about it. Klaviyo's own community documents a case where importing a new list can override existing suppressed and unsubscribed profiles. A clean import of a customer file, run after suppressions were set up correctly, can undo the work.
The cost is immediate. Klaviyo's guidance is to keep spam complaint rates well below 0.1%, and previously suppressed recipients are the single group most likely to complain, because they already decided they did not want your email.
The complaint thresholds are now enforced, not advisory
Since 2024, Google and Yahoo apply formal requirements to anyone sending roughly 5,000 or more emails a day. The numbers that matter:
- 0.3% spam complaint rate is the enforcement line. Cross it and mail starts being rejected outright at the SMTP level, not filtered to spam. It simply does not arrive.
- 0.1% is the working target. Google's guidance treats staying under 0.1% as the condition for reliable inbox placement. The gap between 0.1% and 0.3% is warning track, not safe ground.
- Authentication is mandatory. SPF, DKIM, and a published DMARC record on your sending domain, with alignment passing.
- One-click unsubscribe is mandatory on commercial sends, via the RFC 8058 List-Unsubscribe header. Klaviyo handles this when configured correctly, but a replatform is exactly the moment a working configuration silently stops being one.
A mailing-to-suppressed-profiles mistake that would have cost you placement in 2022 can now cost you delivery entirely. That is why the pre-send checks below are worth an afternoon.
The other three causes, in order
Authentication was reconfigured and not fully verified
A replatform often coincides with DNS changes. If DKIM, SPF, or your branded sending domain were rebuilt and not verified end to end, you are sending as an unauthenticated stranger from a domain mailbox providers recognize. Under the current sender requirements this is no longer a soft penalty. Check DMARC alignment explicitly: a DMARC record that exists but fails alignment passes a casual inspection and fails the actual requirement.
Warm-up was based on volume rather than engagement
The common mistake is ramping send volume on a schedule instead of stratifying by engagement. The correct sequence is concentric rings: start with people who clicked in the last 30 days, hold for several sends, widen to 60-day clickers, then 90, watching complaint rate at each ring. Volume-based warm-up sends to everyone sooner and teaches providers the wrong thing about your list.
One measurement trap here: do not judge the warm-up on open rates. Apple's Mail Privacy Protection pre-fetches tracking pixels on delivery, which registers false opens in Klaviyo and inflates open rates by roughly 15 to 35% depending on how much of your list reads on Apple Mail. A warm-up that looks healthy on opens can be failing on the metrics providers actually see. Judge rings on click rate and complaint rate.
Dormant profiles came back to life
A wholesale import flattens engagement history. Profiles that had gone quiet for eighteen months arrive looking like ordinary subscribers, and your first campaign reaches an audience far larger and far colder than the one you were mailing before the move.
What to check before the first send
- Count suppressed records in the old platform and in Klaviyo, then compare the totals. A meaningful gap means something did not transfer.
- Spot-check specific addresses. Take five known unsubscribes and five known hard bounces and confirm each shows as suppressed in Klaviyo.
- Re-verify suppression counts after every import, not just once. This is the step that catches the override problem.
- Confirm SPF, DKIM, and DMARC are published and aligned, not merely configured. Send a test to a Gmail address and inspect the headers: you want spf=pass, dkim=pass, dmarc=pass.
- Confirm the one-click unsubscribe header is present on a real campaign send, not assumed from settings.
- Rebuild engagement segments from imported timestamps before sending, so recency reflects real behavior rather than import date.
- Set up Google Postmaster Tools on your sending domain before launch, so you have provider-side complaint and reputation data when you need it.
How to recover if you already sent
Stop broad campaign sends immediately. Reduce your audience to people who clicked in the last 30 days and mail only them for two to three weeks. This is not a growth strategy, it is a repair: you are giving mailbox providers a run of clean engagement data to overwrite the bad signal.
Reconcile suppressions while that runs. Then widen the audience in stages, watching complaint rate at each step rather than waiting for open rate, which is both slow and inflated.
Watch Postmaster Tools during recovery. Domain reputation there moves through rated tiers, and the climb back is slower than the fall. A bad week of sending can take a month or more to undo, which is the strongest argument for running the checks above before the first campaign rather than after.
Frequently asked questions
How long after a replatform does deliverability usually drop?
Two to three weeks is typical. The first sends often look fine because reputation is carried over, and the decline appears once complaint and non-engagement signals accumulate.
What spam complaint rate gets email blocked entirely?
0.3% at Gmail and Yahoo is where rejection begins for bulk senders. Treat 0.1% as the operating ceiling. These are enforced thresholds, not guidelines.
Does a new sending domain always require a warm-up?
Any material change to sending identity or volume warrants one. Stratify by engagement in expanding rings rather than ramping raw volume on a calendar.
Why do our open rates look fine while revenue falls?
Apple MPP registers opens on delivery for Apple Mail users whether or not anyone read the email. Open rate can hold steady while actual inbox placement decays. Use click rate as the primary engagement measure.
Can we import our old list wholesale if it was all opt-in?
Opt-in status is not the issue. Engagement recency is. A three-year-old opt-in who has not opened anything in two years still damages placement.
Sources
- Klaviyo Community. Imports overriding existing suppressed and unsubscribed profiles.
- Klaviyo Help Center. How to reduce spam complaint rates.
- Klaviyo Help Center. Email deliverability best practices reference.
- Mailgun. Google and Yahoo bulk sender requirements.
- Klaviyo Help Center. Identifying Apple Mail Privacy Protection opens.
- Email Industries. Suppression lists during platform migrations.