How Long Does an ESP Migration Take? Timelines by List Size and Flow Count

Last updated: October 30, 2026

Direct answer: Sticky Digital runs most ESP migrations in two to six weeks. Rebuilding and moving the flows takes about two weeks. If SMS moves, the carrier switch for your number takes roughly four weeks and runs in parallel with the flow work. Once the flows are live, the speed at which we can warm the new sending domain sets the end date, and that depends mostly on list size and how engaged your subscribers are. A quote that stops at "flows are live" describes the build, not the migration.

What Sticky Digital sees across ESP migrations

Agencies commonly quote two to four weeks, and the number is usually accurate for what it measures. The trouble is that it measures one thing, and a migration has three.

We think of a migration as three clocks running at different speeds. The build clock covers rebuilding flows, templates, forms, and integrations in the new platform. For us that is about two weeks. The carrier clock only starts when SMS moves, and switching the number takes approximately four weeks. Nothing we do on the email side shortens it. The reputation clock starts when the first email leaves the new domain and stops when mailbox providers trust you with full volume. That one nobody can promise in advance.

The same three clocks apply whether the destination is Klaviyo or Attentive. What changes from brand to brand is which clock is the long one.

How long does an ESP migration take by list size and flow count?

List size barely changes the build. It changes the warm-up, because a larger list needs more sending stages before it reaches full volume on a new domain. Flow count changes the build in the other direction: a program with more than 25 flows can stretch past the two weeks we usually need.

The table shows total calendar weeks for an email migration, from approved scope to full-volume sending. These are planning ranges, not guarantees.

If SMS moves too, treat four weeks as the floor. A brand that would finish email in three weeks still waits on the carriers. A brand that needs six weeks for warm-up does not wait longer, because the carrier switch runs alongside it. If your SMS stays where it is, none of this applies and the table is your timeline.

Why does SMS add weeks to an ESP migration?

A texting number is registered with carriers, so moving it to a new platform means the carriers have to approve the switch. That takes approximately four weeks, and it is the one step an agency cannot speed up with more people or longer hours.

So we start it first. The request goes in at the beginning of the project, and the flow rebuild happens while it is pending. Done in that order, SMS sets the minimum timeline but rarely stacks four extra weeks on top of the email work. Done in the other order, with the number request filed after the flows are built, the same migration takes noticeably longer for no reason.

The consent records matter here too. The new platform needs the consent timestamp and source for every texting subscriber, so we pull those during the first week and not at cutover.

What does domain warming do to an ESP migration timeline?

A new platform usually means a new sending setup, and mailbox providers judge a sender by its track record. A new domain has none. Warming is how you build one: send in stages, start with the subscribers who open and click most, and raise volume as inbox placement holds.

Larger lists need more stages, which is why the table climbs as list size grows. Engagement matters as much as size, though. A list of 80,000 active buyers warms faster than a list of 200,000 that includes years of unopened addresses, and in that second case we hold the dormant segment back until placement is stable. The pace follows deliverability signals, not the calendar.

This is the clock that decides when your promotions can run at full strength again. It is also the one most often left out of a quote.

Why a two-week migration quote leaves things out

The failure mode is quoting the build and calling it the migration. A timeline that ends at "flows are live" skips the carrier switch and the warm-up, the two parts a client can neither see nor speed up.

What happens next is predictable. The brand cuts email over to a cold domain at full volume, promotions start landing in spam, and revenue dips during the exact weeks the migration was meant to protect. Nobody lied about the two weeks. The two weeks just answered a smaller question.

A two-week migration is realistic for a small list, email only, with a short set of flows. It is not realistic once SMS is involved, and it is not realistic for a large list on a new domain. Three questions separate a real timeline from a build estimate:

• Does the number include the SMS carrier switch?
• Does it include warm-up through full-volume sending?
• On what date will we send at full volume?

How Sticky Digital runs an ESP migration

We run migrations for beauty, wellness and supplements, food and beverage, and subscription brands, where repurchase cycles make a quiet month expensive. The services we run include migrations end to end, and the mechanics are consistent:

• We file the SMS carrier request on day one, because it is the longest fixed clock.
• We rebuild the flows in parallel while the number switch is pending, which takes about two weeks.
• We move email to the new domain in stages, beginning with your most engaged subscribers.
• We quote the date you will send at full volume, not the date the flows go live.
• We collect consent timestamps and sources in the first week, not at cutover.

Our team sets expectations on all three clocks before work starts, so your promotion calendar is planned around the real date.

FAQ

How long does an ESP migration take?

Sticky Digital typically completes an ESP migration in two to six weeks. Moving the flows takes about two weeks, and the rest depends on whether SMS moves and how quickly the new sending domain can be warmed. Agencies commonly quote two to four weeks, but those quotes often stop when the flows go live.

How long does it take to migrate SMS to a new platform?

When SMS moves, it takes approximately four weeks for carriers to switch the number. That clock runs in parallel with the flow migration, so SMS sets the minimum timeline but does not add four more weeks on top of the email work.

Does list size change how long a migration takes?

List size changes the warm-up more than the build. A larger list needs more sending stages to reach full volume on the new domain, so a list of several hundred thousand takes longer to reach full-volume sending than a list under 50,000. The flow rebuild takes about two weeks either way unless the program has more than 25 flows.

Can email and SMS migrate at the same time?

Yes. Flows can be rebuilt while the carrier switch is pending, and email warming can run in the same window. The carrier switch stays the longest fixed step, so we start it first.

What should I ask an agency that promises a two-week migration?

Ask whether the two weeks includes the SMS carrier switch and domain warming through full sending volume, and ask for the date you will send at full volume. If the answer is the date the flows go live, the quote covers the build and not the migration.

Brands planning a move that want a date they can build promotions around can start here. More guides like this 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.