ESP Migration: A Complete Guide to Switching Email Providers Safely in 3 Months

Thinking about switching email platforms (aka ESPs)? You’re not alone.

Does the thought make you feel overwhelmed? You’re still not alone.

Migrating from one ESP to the next can be incredibly daunting. For one, ESP migration is not like switching websites, where one day your URL points to the old site and the next day it points to the new one. There’s no switching ON/OFF. And there’s certainly no big “migrate” button you could use. Which is why we tell our clients to estimate a 3-month migration period.

As email marketing consultants, we’ve performed dozens of migrations for our clients so we have some concrete experience navigating this across a number of different tech stacks.

Now we’re sharing common pitfalls so you can plan for a successful migration where your deliverability remains healthy, your operations seamless, and your sanity intact.

Common ESP Migration Mistakes

When it comes to migrating ESPs, timing and execution is everything. It can make or break your results (and your business). If email is a major revenue driver for you, it’s critical to approach your migration carefully. Here are the top three ESP migration mistakes to avoid.

1. Not Factoring in Warm-up Time

One of the biggest mistakes when migrating to a new ESP is treating it like the flick of a switch. Jumping into a new ESP from one day to the next and emailing your full list without warming up the new infrastructure is a surefire way to create serious deliverability issues.

ISPs (like Gmail, Yahoo, and Outlook) monitor new senders closely. You might think “Oh, the email address we use is the same so everything should be fine.” However, it’s important to note that when you’re sending from a new ESP, you’re essentially sending from a new infrastructure so the ISPs still count your email address as “new”—making it susceptible to being marked as spam. If you send too soon or too much, it raises red flags, tanks your sender reputation, and negatively affects your deliverability.

So, what is warm-up time?

In technical terms, it’s called “warming up the new infrastructure.” It’s the process of gradually building your domain’s (or IP’s) reputation by sending small volumes of email to highly engaged recipients first. Over time, you increase sending volume as you prove to ISPs that your emails are wanted and valuable.

It’s best practice to migrate and send emails that are more likely to get positive engagement. For most businesses, the best place to start is your welcome flow or something that gets really good open rates like an internal team newsletter. It works because:

1. It normally has high open and click-through rates
2. It’s sent to people who requested to hear from you (less risk of spam complaints)

Warming up simply means sending a few campaigns that you know will get good engagement before you begin sending larger campaigns to larger lists. Depending on your email program, your warm up period can last between 1-4 weeks.

2. Picking a Single Switch Over Date

It’s tempting to think of ESP migration like a website migration where you can move everything from the old system to the new one overnight.

Unfortunately, email doesn’t work like that. Treating it like a one-day event puts both your deliverability and your revenue at serious risk.

We already discussed why you need to warm up your infrastructure first. But secondly—and just as importantly—you risk disrupting your flows that your customers may already be halfway through.

Think about your post-purchase emails, nurture series, onboarding emails, or abandoned cart flows. These flows are where for many businesses most email revenue comes from. In fact, according to recent reports like Omnisend’s 2025 Ecommerce Marketing report, email flows generate over 30% of email revenue despite making up only 2% of sends.

If you pick a single migration date and simply move everything over, you could accidentally:

• Leave customers stuck mid-journey without finishing flows
• Interrupt your customer journeys or cause duplicate emails received
• Lose valuable revenue

Email flows are triggered by events (like purchases, signups, cart abandonments) coming through your integrations. When you switch ESPs, historical data usually doesn’t transfer over, but if it does, those old events won’t necessarily trigger flows on the new system. Usually it’s new real-time events that will. This makes an appealing case to design a “from this point onwards” approach to migrating flows.

We call this a staggered migration where you decide to let what’s already in motion run its course on the old ESP and start with new events triggering new flows on the new ESP.

While it is possible to migrate people mid-flow, we find it’s unnecessarily complex and full of risk to attempt that. You risk confusing customer messaging, and you risk your brand reputation and revenue.

3. Not Factoring in Enough Time

It goes without saying, nobody wants a migration project to drag on longer than it has to. If your boss thinks you can wrap up an ESP migration in one month, why push for three?

Because the real cost of rushing isn’t the time but the lost revenue. If you skip warming up your infrastructure it’s possible that you’ll see a drop in attributed email revenue drop. Then consider those who are mid-flow that may not receive all their intended emails and the lost revenue from that.

Damaged deliverability can sometimes take much longer to fix (sometimes longer than the 3 months entirely).

Instead, consider planning for a well-executed 3-month migration where domain warm up happens gradually and flows are transitioned carefully. This approach will also give your team time to adjust to the new platform which is critical to the success of your transition.

If getting buy-in is a sore point, take a look at your metrics, particularly revenue generated by flows and campaigns. Then, put a dollar amount to the risk. If you can show leadership that rushing migration could cost the business a significant loss in revenue, it’s a lot easier to get leadership on board for a slower, more thoughtful migration strategy.

Our 3-Month ESP Migration Plan

Now that we’ve covered the biggest mistakes to avoid, let’s walk through the 3-month migration framework we use with our clients for a smooth, low-risk migration.

Month 1: Integrations & Key Flows

In Month 1, your focus is setting up the technical foundation and starting the warm-up process in a way that doesn’t disrupt your current email revenue.

Spend some time configuring your new ESP.

• Set up all necessary integrations
• Configure tracking settings
• Authenticate your sending domain
• Migrate your subscriber lists
• Re-create key segments
• Set up user profiles and admin privileges

Move over your opt-in forms and key flows:

• Welcome series (whether it’s a coupon code or download)
• If you’re in ecommerce move over browse abandonment & abandoned cart
• If you’re not in ecommerce move over your contact forms or any embedded forms

Make sure your new ESP is fully connected to your website and events are being tracked properly. Also, take the time to check that your site’s privacy policy and consent terms are up to date with your new tech stack adjustments. Sometimes minor adjustments are needed depending on the new ESPs functionality.

This phase is mostly setting up integrations and turning on high-engagement flows that are ideal for warming up your new sending domain.

During this phase:

• Leave your other flows running in your old ESP for users who are already in them.
  For example, if you turn on a new signup form in your new ESP, new subscribers will flow into the new system naturally but people already moving through the old welcome series should be allowed to complete the old welcome series before you turn it off.
• Adjust to sending to only highly engaged groups (this is not the time to send broad campaigns from the new system.)
• Yes, you will have double systems running. Meaning, events from your website will be going into BOTH the new and old ESP. This is good for checking the accuracy of the installation and that events, segments, flows, and reporting are consistent.

Month 2: Re-create Segments & Campaigns

In Month 2, it’s time to start migrating your broader campaigns and any associated lists so you can officially send from your new ESP and stop sending campaigns from your old ESP.

It can be a delicate balance to migrate segments – what used to be dynamic in your old ESP may be a combination of static + dynamic in your new ESP because the data may not exist yet for a fully dynamic segment.

For example: 90-day engaged segment

1. Export a one-time static list from your old ESP of this segment.
   (Example: 90-day engaged segment as of [today’s date])
2. Import the static list to use as a starting point for this segment in your new ESP.
3. Set up a new 90-day engaged segment in your new ESP that references the static list + dynamic metrics like has opened an email or visited your website in the last 90 days.

This can be your new 90-day engaged segment until you’ve got 90 days worth of data, at which point you can remove the static list reference.

This is good enough to safely start sending campaigns from the new ESP while your dynamic segments have a chance to populate based on new data sources over time.

Migrating campaigns usually means recreating your email templates from scratch, including any universal blocks or custom tracking settings.

With the segments & campaign templates, you should be able to start sending campaigns from the new ESP.

Month 3: Complete the Move

Now’s the time to bring over the rest of your flows:

• Post-purchase flows
• Winback series
• Replenishment or review requests
• Any other flows you might have.

At this stage, you are shifting weight with your flows. That means you’re letting new people enter new flows & keeping an eye on old flows so that when people have finished receiving them they get turned off.

Now’s also the time to migrate over other segments you used, possibly broader ones.

• 90 day, 180 day, 365 day
• Full list (depends a little on your new ESPs billing system if it makes sense to bring everything over or clean it before moving people over)

At the end of this phase you should be operational. You should go into your old ESP and see what can be turned off, archived, account settings adjusted, and billing stopped.

Post-migration

Congratulations, you’re now running your email marketing out of a new ESP!

For our clients, we usually spend some time every few weeks to check what needs adjusting: segments, reporting, flows, etc. There will be some minor adjustments after the fact, but the bulk of the migration is complete and now you just transition to the usual email optimization strategy.

Example: 180-day opens will gradually become 180-day opens on the new platform dynamic segment and not connected to a static list import

 At some point you’ll want to actually export your data and turn OFF the old system. Usually, because you have no active flows and campaigns you can stop paying for it. We recommend keeping the account active for 1-6 months just in case, especially if billing can be adjusted so that it’s free. It can still be a great resource for past campaigns and performance, though a good idea is to pull that information out so you have the reference and insights to inform future strategies.

A smooth ESP migration isn’t about speed, it’s about strategy. The goal is to protect your deliverability, maintain a seamless customer journey, and come out the other side stronger than before.

If you’re planning a migration, take the time to plan a phased rollout.

Having navigated numerous ESP migration projects, we know it can be a tedious process. Which is why at Centric Squared, our approach is strategic, systematic, and tailored to the needs of our individual clients. If an ESP migration is the next step in achieving your business goals, give us a call today.

Common Migration FAQs