How to Switch Email Platforms Without Hurting Deliverability

Last updated October 28, 2026.

Direct answer: Sticky Digital recommends running a platform switch as three tracks that all start before the cutover date. Rewarm your active contacts on the new platform for two to eight weeks, request the release of your SMS short code the day you decide to move because the transfer takes four to six weeks, and export subscribed and unsubscribed contacts as two separate files. Decide up front whether you will rebuild every flow as it is or redesign as you go, and do not cut over in the weeks before Black Friday. The sequence is the same whether you are moving to Klaviyo from Attentive, Postscript, or another email platform, or moving the other way.

What changes when a brand switches email platforms

Sticky Digital has run email and SMS for DTC brands since 2019 and is a Klaviyo Platinum Partner. Our roster includes beauty brands such as Murad, U Beauty, and ZO Skin, and clients usually see 25% to 40% of store revenue coming from email within six months. That revenue is exactly what a careless switch puts at risk, which is why we treat a migration as a deliverability project before it is a design project.

The setup that causes trouble is a cutover date chosen by something other than readiness. A contract end date, a promotion, an internal deadline. None of those mean anything to Gmail.

Inbox providers judge a sender by how recipients treat its mail. They do not care how well known the brand is. Klaviyo’s own training material says a new sending domain is a blank slate to inbox providers however established the brand is. Your customers know you. Gmail has never met you.

Why should you rewarm active contacts first when you switch?

Because a new platform means a new sender in the eyes of the inbox providers, and a new sender earns trust by being wanted. Klaviyo’s help center says any new sender must warm up its sending infrastructure when it moves to a new email service provider. The method is simple to describe and tedious to run: start with the people most likely to open and click, and widen the audience only as the results hold.

Klaviyo’s guidance for the first two weeks is to send campaigns to the 30-day engaged segment, then broaden to the 60-day segment if engagement stays high and the audience does not jump in size. Its training course adds the gates: do not move to the next volume tier until bounce rate is under 0.5% and spam complaints are close to zero.

Our range is two to eight weeks. Where a brand lands depends on how large the list is and how quickly the numbers hold at each step. Turn on your highest-engagement flows early, welcome and post-purchase first, because those reach people who just did something and expect to hear from you.

Set up Google Postmaster Tools for the sending domain before the first send. Google’s guidance is to keep the reported spam rate under 0.10% and never reach 0.30%, and you cannot manage a number you are not watching. The bulk sender rules apply to anyone sending roughly 5,000 or more messages a day to Gmail addresses, which describes most $10M+ brands.

How long does an SMS short code transfer take?

Longer than the email side, and it is the part most teams start too late.

Klaviyo’s help center puts a short code port at about four weeks, with long codes and toll-free numbers at two to four. In our experience a short code runs four to six. Postscript’s documentation shows how much the range moves by direction and hosting: it says a short code can move from Klaviyo to Postscript in as little as two business days when the code is hosted on Twilio, and one to two weeks when it is not. You will not know which case you are in until you ask the provider that holds the number, so ask first.

The old provider has to approve the release. That approval, not your own paperwork, is usually what sets the date. Request it the day the decision is made, and plan the SMS cutover as its own date, separate from email.

If you are moving SMS subscribers from Attentive, request the export early too. Klaviyo’s import guide tells you to plan for Attentive’s turnaround time. For the US and Canada, Klaviyo’s migration guidance says not to collect consent or message anyone until you have a short code or a verified toll-free number.

How do you move subscribers without emailing the people who opted out?

Ask your current platform for two exports, not one: everyone who is subscribed, and everyone who unsubscribed or bounced. Keep them as separate files from the start.

In Klaviyo, the unsubscribe file goes into suppressed profiles. Klaviyo’s migration article walks through it: prepare a file with one email per row, open the suppressed profiles view, and upload it. Load that file before you import subscribers. The reverse order leaves a window where an opted-out customer sits in your account as an ordinary contact.

Two more details matter. Carry over the original consent timestamp for each subscriber rather than stamping everyone with the import date. And turn off any flow triggered by the list you are about to import, unless you want every imported contact to enter it. Klaviyo’s onboarding course flags the second one, and it is easy to forget when the import is the last task of a long week.

Should you rebuild every flow exactly as it is, or redesign during the switch?

Rebuild one to one first, unless the flows were already broken. The reasoning is about what you can learn. If revenue per recipient drops after a platform change and you also changed the flow logic, the offers, and the creative, you cannot tell whether the platform, the warm-up, or the redesign caused it. Change one variable at a time.

Redesign makes sense for a flow whose structure no longer matches the business, a replenishment sequence for a product you have since restocked in different sizes, say. In that case redesign that flow and migrate the rest as is. Run the redesigns after the warm-up is done, when the sending reputation is steady and the results mean something.

Can you switch email platforms right before Black Friday?

No. Add up the timelines. Warm-up takes two to eight weeks and a short code transfer takes four to six, and the two run in parallel. In 2026, Black Friday is November 27. A brand that has not started warming by now should plan the move for January.

The heaviest send volume of the year is the worst time to build a sender reputation. Every step of a warm-up is supposed to be gated on clean results, and a promotion calendar does not wait for a gate. If the old platform’s contract ends before the holiday, ask for a short extension. That conversation is cheaper than a cold sender at peak volume.

Why platform switches hurt deliverability: the full list goes out on day one

The specific failure is a cutover-day send to the entire list from a sender with no history, with the unsubscribe file missing or loaded late.

Two things go wrong together. The unengaged addresses, the ones who have not opened in a year, get mail from an unfamiliar sender, and some of them hit the spam button. The opted-out customers who slipped through the import hit it too, and they are the most likely to. Add the two groups up against Google’s ceiling. On a 200,000-person send, a 0.3% complaint rate is 600 complaints, and the guidance is to stay under 0.10% and never touch 0.3%. A cold sender mailing an old list can get there in a single afternoon.

Recovery takes longer than the mistake did. The brand then warms up anyway, but now from a damaged position, during the weeks it needed for the actual launch.

How Sticky Digital runs a platform switch

We build the plan around the slowest track and let the calendar follow it. In practice that means five things.

1. The warm-up starts with engaged segments on the new platform and widens only when the previous step’s bounce and complaint numbers are clean.
2. The SMS number release is requested on the day the switch is decided, and the SMS cutover gets its own date.
3. Subscribed and unsubscribed contacts move as two files, unsubscribes first, with original consent timestamps carried over.
4. We decide flow by flow whether to rebuild one to one or redesign, and default to one to one.
5. The cutover date comes from the warm-up and number timelines, and it stays clear of Black Friday. Our services page covers how migrations fit into a retention engagement.

FAQ

How long does it take to switch email platforms without hurting deliverability?

Plan on two to eight weeks of warm-up on the new platform, sending to active contacts first. If an SMS short code has to transfer, add a parallel track of four to six weeks. The longer track sets the cutover date.

Do you have to warm up a new email platform?

Yes. Klaviyo’s help documentation says any new sender must warm up its sending infrastructure when it moves to a new email service provider. Inbox providers treat the new setup as a new sender regardless of how established the brand is.

What should you do with unsubscribed contacts when you migrate?

Export them as a separate file from your subscribed contacts and upload them to the new platform’s suppressed profiles before you import anyone else. Carry over each subscriber’s original consent timestamp. Never re-import an unsubscribed contact as subscribed without fresh consent.

Can you switch email platforms during Black Friday and Cyber Monday?

Sticky Digital advises against it. A warm-up needs two to eight weeks of gated, gradual sending, and the holiday calendar sends the largest volumes of the year. A brand that has not started warming before the holiday should switch in January.

Does the same process apply when moving between Klaviyo, Attentive, and Postscript?

The sequence is the same in every direction: warm active contacts first, request number releases early, and move unsubscribes as their own file. The timelines differ by provider and by where your SMS number is hosted, so ask the provider that holds the number for a date before you commit to a cutover.

Brands that want a platform switch planned and run end to end can start here. More retention guides live on the Sticky Digital blog.

Article By: Mariel Kilroy, Co-Founder, Sticky Digital

Mariel Kilroy is the Co-Founder of Sticky Digital, a retention marketing agency specializing in email, SMS, loyalty, and subscription growth for DTC brands.