12/2025

How to Successfully Migrate ESPs (and What your Boss Needs to Know)

Before you dig in, why don't you get access to RE:markable

RE:markable is the weekly email about emails. Dropping the latest email marketing news, updates, insights, free resources, upcoming masterclasses, webinars, and of course, a little inbox mischief.

Subscribe to RE:markable

You cannot just flick the switch from “Old ESP” to “New ESP” and carry on as normal.

Technically, you can, I’ve done it in past roles when I was very junior (why was this even my responsibility!?) but the cost is super brutal. It ruins deliverability, tanks engagement, causes panicking stakeholders and can be months (or years) of recovery work and lots of money to get you back on track.

This blog is your guide to:

Why you can’t just switch providers on and off
What actually changes under the bonnet when you migrate
Why deliverability is obsessed with change and how moving ESP impacts it
How IPs, IP pools, and “new sender” status really work
What a proper warmup and migration should look like
Why you must run a deliverability audit before you move
How to communicate all of this to leadership so you get the time and resources you need

This is the blog you send to your boss when they say:

“Can’t we just move everything over by the end of the month?”

1. Why you can’t just switch ESPs on and off

I’ll start with a confession (don’t you just love a confession time)...

Early in my career, I did “lift and shift” ESP migrations with zero strategy. No warmup, no deliverability checks and no gradual transition.

I killed email that way. 10/10 do not recommend.

Here’s the key mindset shift:

Deliverability isn’t about “good” or “bad”. It’s about change.

You might be making a really positive internal change - say you’re moving from Mailchimp to HubSpot (I love HubSpot so much), or to a more enterprise-grade ESP. Great for your team, your data, your attribution, your reporting.

Inbox filters do not care that you’re happier or if it’s best for the business.

What inbox providers (Gmail, Outlook, Yahoo, corporate filters etc.) see is this:

A new IP sending your email
A change in sending patterns and volumes
New infrastructure and authentication
Possibly different sending domains or tracking domains

And inbox filters are hardwired to be suspicious of sudden change. Sudden spikes in volume or new IPs are classic spammer behaviour, so filters treat all big changes as “guilty until proven innocent”.

So when leadership says, “We’re just changing platforms”, what the inbox hears is:

“We’ve completely changed the pipes we send from, please re-evaluate us from scratch.”

That’s why you cannot just switch off one provider on Friday and switch on the new one on Monday and expect smooth sailing.

2. What actually changes behind the scenes when you switch ESPs

From your boss’s perspective, not much changes:

The brand is the same
The audience is the same
The content probably looks similar

So why is it so fragile?

Because under the hood, a lot shifts at once:

2.1. Your sending IP (and IP pools)

Every marketing email is sent from an IP address. Inbox providers track behaviour at IP level (and domain level) and build a reputation for each sender over time.

When you move ESPs, a few things can happen:

You move onto different shared IPs
You’re assigned a new dedicated IP
You’re put into an IP pool reserved for new customers

Most ESPs have different pools for:

Brand new customers (“we don’t know you yet”)
Established, high-volume, good-reputation senders
High-risk/“we’re not sure about you” traffic

When you first land on a platform, you’re not automatically on the “gold” IPs, even if you’re paying a lot. You usually start life on:

Experimental / onboarding IP pools

Those pools are full of other senders who are also “new”, which means:

You’re sharing IP reputation with people you’ve never met
If some of them are abusing lists or spamming, you share the consequences

This is why “cheap” ESPs are often a deliverability risk: it’s easier for low-quality senders to sign up, and protections on those IP pools can be weaker.

Day-to-day, once you’re established, IP details fade into the background. But during a migration, IPs and which pool you’re placed into are absolutely critical.

2.2. Sending patterns, volume, and cadence

You might think,

“We send ~100k emails a week now, we’ll send ~100k emails a week from the new ESP. What’s the issue?”

The issue is suddenness.

Inbox providers are looking for:

Sudden jumps in volume from a new IP
Sudden changes in who you’re sending to
Sudden changes in your engagement profile (opens, clicks, complaints, bounces)

If yesterday that IP was sending 0 emails and today it’s sending 100k, that is a massive red flag. IP warmup is how you avoid that.

3. Why your numbers tank after a migration (and why it’s not always bad news)

Quick detour, because this creates panic.

Another reason you cannot “flip the switch” is metrics whiplash. Your open and click rates will almost certainly change, and often they look worse.

Sometimes that is because:

Your new ESP does stricter bot filtering
It counts opens and clicks differently
It removes pre-fetching and security-scanner activity

In other words: your old numbers were inflated, and the new platform is giving you a reality check. I’ve already written the whole breakdown in your blog Why Your Opens and Clicks Have Dropped After Switching Email Platforms here.

So when you’re reporting to leadership, you need two messages:

“Some of the drop may just be more honest tracking - not failure.”
“But we also need to proactively protect deliverability, because that can fall off a cliff if we rush.”

4. Deliverability is obsessed with change

Let’s spell this out in non email friendly language.

When you change ESPs, inbox providers see changes in:

IP address(es) sending your email
ESP infrastructure (how the mail is routed)
Authentication records (SPF, DKIM, DMARC)
Sending volume patterns
Sending domains / tracking domains

Any one of those can trigger extra scrutiny. Changing all of them at once? That’s a full-scale investigation.

Filters ask:

“Is this the same sender I trusted last month?”
“Do people still seem to want these emails?”
“Have complaints or bounces gone up?”

If they don’t like the answers, they will:

Throttle your volume
Push more of your mail into spam
Block you outright in worse cases

That’s why the migration strategy is not about convenience; it’s about managing the amount of change inboxes see at any one time.

5. What is an email/IP warmup, really?

A warmup is not:

Sending a couple of test campaigns and hoping for the best
Using an automated third-party warmup tool and calling it done
Splitting your list into random chunks and blasting each chunk in order

An email warmup is:

The process of gradually, not suddenly, increasing email volume from a new or rarely used IP so inbox providers can learn to trust it and you.

It matters when:

You move ESPs and start using a new IP
You’re given new IPs on the same ESP
You’re relaunching after a long period of inactivity
You’re shifting from very low volume to much higher volume

New IPs and “cold” domains are suspicious by default. Warmup is your “prove we’re legitimate” phase.

6. Warmup is per inbox provider, not just per list

This is the bit I really want you and your boss to understand.

Most “DIY” warmups go like this:

Take your list of, say, 50k
Chop into five chunks of 10k
Send to chunk 1, then 1+2, then 1+2+3 etc.
Call it a warmup

Sounds fine on paper. But no, absolutely not.

Chunk 1 might be:

80% Outlook
10% Gmail
10% “other”

Chunk 2 might be:

Mostly Gmail
Very few Outlooks

Inbox providers do not talk to each other. Outlook has no idea how Gmail sees you, and vice versa. Each provider is building its own picture of your behaviour.

That means:

You can be perfectly warm with Outlook
…while Gmail still treats you as a suspicious new sender

So you can’t just split your list randomly and expect a coherent warmup.

6.1. How to do provider-based warmup properly

The right way to do this:

Run your list through an email validator/enrichment tool (I use Zerobounce).
- Not for cleaning only, but to identify the inbox provider for each address
- You want to know: Gmail vs Outlook vs Yahoo vs corporate domains, etc.
2. Create segments per provider, e.g.:
- Gmail users
- Microsoft/Outlook/Hotmail
  Yahoo
- Apple/iCloud
- Major regional inbox providers if relevant

Build a warmup plan per provider, for example (illustrative only):
- Day 1–3: 100 Gmail + 100 Outlook + 50 Yahoo (most engaged users only)
- Day 4–7: 250 Gmail + 250 Outlook + 100 Yahoo
  Day 8–14: 500–1,000 per day per provider, still biasing towards engaged
- And so on, gradually increasing

The actual numbers depend on:

Your historic volume
Domain age and reputation
Current deliverability issues (or lack of them)
How engaged your list is right now

But the principle is the same:

Each inbox provider needs its own gradual warmup curve.

This is why warmup is real work, not a tick-box exercise. It takes planning, segmenting, and daily monitoring.

7. Before you move, you MUST run a deliverability audit

This is non-negotiable if you want to keep your sanity and email performance.

If you already have a deliverability problem on your current ESP and you move without fixing it, you do not get a clean slate. You just:

Move the problem into a new environment
Make it harder to diagnose
Slow down recovery because now the IP and the domain look risky

A deliverability audit is a structured review of everything affecting whether your emails land in inboxes or spam.

A proper audit looks at things like:

Inbox placement (how much is going to spam vs inbox)
IP reputation and any blocklists
Domain reputation
Authentication (SPF, DKIM, DMARC)
Sending patterns (volume, cadence, spikes)
List hygiene and spam complaints
Engagement minus bot activity (actual humans)

For your boss, translate this into business language:

“We need to know whether our current email performance is built on solid ground or we’re about to rebuild a wobbly house on a new plot.”

If the audit shows issues, fix as much as you can before you move. That might mean:

Cleaning the list
Fixing authentication
Adjusting frequency
Removing problematic segments or sources

Then you migrate from a stable baseline, not from an already suffering problem. And remember, if your deliverability is bad now it will get even worse if you move ESP without the right strategy.

If you need help with the audit piece…

A quick sidebar here, because the deliverability audit step is where most teams realise they either:

don’t know what “good” looks like, or
don’t have the confidence to interpret what they’re seeing.

If that’s you, you’ve got two options:

1) Learn to do it properly, end to end

I run live Deliverability Masterclasses where I teach you exactly how to audit, test, diagnose, and improve your sending health — the real, practical version you can apply the same day.
You can view upcoming masterclasses here.

2) Get the audit done for you

If you’d rather have expert eyes on your programme before you migrate, I offer full deliverability audits as a service. You can get in touch here.

Either way: do not skip this stage. A clean deliverability baseline is the difference between a smooth migration and a six-month recovery project you didn’t plan for.

8. What a good ESP migration actually looks like

Let’s walk through an idealised version. You’ll adapt it for your scale, but the phases are the same.

Phase 1: Pre-migration planning

This is where you live for a while. Rushing this stage is how strategies die!

8.1. Set clear objectives and constraints

With leadership, agree:

Why you’re migrating (cost, features, integrations, support, compliance, etc.)
What “success” looks like 3–6 months post-migration
What is not acceptable (e.g. losing critical automations, breaking billing emails)

8.2. Inventory everything in your current ESP

Literally everything:

Campaign types (newsletters, promos, events, product updates)
Automations and flows (welcome, onboarding, cart/browse abandonment, reactivation, NPS, lifecycle journeys)
Transactional vs marketing sends (and where they live)
Segments, tags, custom fields, and scoring models
Templates and content blocks
Forms, preference centres, and double opt-in flows
Integrations (CRM, ecommerce, webinar tools, payment platforms, etc.)
Suppression lists (unsubscribes, bounces, complainers)

You need to map this so you don’t accidentally switch off something critical during warmup.

8.3. Stakeholder map and comms pla

This is the bit your boss needs to know

Stakeholders that must be in the loop:

Marketing leadership
Sales/RevOps
Product/growth teams
Customer support
IT/security
Legal/compliance (if you’re in regulated industries)

They need to understand:

There will be a warmup period
You will not be able to “just blast the whole list” from day one
There may be short-term volatility in metrics and capacity

Set expectations early: this is about protecting long-term revenue, not hitting a convenient internal date.

8.4. Technical groundwork

Before you ever send a single email from the new ESP:

Set up domains and subdomains for sending - don’t switch domains please! Use the ones you already have.
Configure SPF, DKIM and (ideally) DMARC for the new ESP and verify it works
Align from addresses with what your audience recognises

Phase 2: Build the foundation in your new ESP

This is the plumbing phase.

8.5. Configure the account properly

Team access and permissions
Default from-names and addresses
Timezone, currency, and account-level settings

8.6. Discuss IP strategy with your ESP

Do not assume they’ll magically put you on the right IPs.

Have a grown-up conversation about:

Expected monthly send volume
Send patterns (daily/weekly/seasonal spikes)
Whether you’re on shared vs dedicated vs hybrid
Which IP pools they plan to use for onboarding you
What warmup guidance they provide

Get this in writing where possible. It saves pain later.

8.7. Migrate data and workflows carefully

Import your suppression lists first so you don’t accidentally email unsubscribed users
Migrate your core lists/segments and check counts match
Rebuild critical automations (welcome, password reset if applicable, core lifecycle flows)
Recreate your most-used templates natively

You might be tempted to import historic reports too, but that’s optional. What matters most during migration is that your current system works.

Phase 3: Warmup and staged cutover

Now we get to the bit everyone wants to skip — and the bit that will punish you the hardest if you do.

Warmup is not optional. It’s not a technical nicety. It’s the mechanism that teaches inbox providers to trust your new IPs, your new sending patterns, and your new infrastructure. If you rush this phase, you will tank deliverability and spend months recovering.

8.8. Decide what stays on the old ESP during warmup

Before you send anything from the new platform, decide which sends remain anchored to the old ESP while you build trust on the new one. Usually, you will:

Keep all mission-critical transactional emails on the stable platform until deliverability is consistently strong on the new IPs.
Move lower-risk campaigns and some simpler automations first, so you’re not gambling with revenue-sensitive journeys.
Only move everything once you’ve seen stable inbox placement across Gmail, Outlook, Yahoo, and major providers.

You can and often should run ESPs in parallel for a while. I recommend keeping the old one live until the new one is fully warmed and stable — not just “sending”.

8.9. Create your warmup plan (the part people skip)

This is the piece that determines whether your migration glides or crashes.

It starts with properly understanding your audience and creating a controlled volume ramp.

Here’s what this looks like in practice:

1) Validate your list

Run your audience through an email validation tool so you know:

Which inbox providers your subscribers use
Which contacts are engaged vs dormant
Which addresses pose risk (role accounts, disposables, potential traps)

You cannot warm effectively without this picture.

2) Segment by inbox provider

You need clean cohorts for:

Gmail
Outlook / Microsoft
Yahoo
iCloud
Regional providers
Corporate domains (if you send B2B)

Each provider learns at its own pace and they do not share reputation signals.
This is why random, linear list-splitting doesn’t work.

3) Build a slow, steady warmup plan

Start with tiny volumes. Think microscopic, not “comfortable”.

For example:

Day 1–3:
→ 50–100 sends per provider to your most engaged subscribers
Day 4–7:
→ Increase by ~50 per day per provider, still using highly engaged cohorts
Day 8–14:
→ Begin adding moderately engaged users, increasing volume gradually
Weeks 3–6:
→ Continue controlled increases, introducing colder segments very slowly and only after deliverability remains stable

This is not a universal ramp but a framework. Your actual numbers will depend on domain age, reputation, historic volume, audience composition, and whether you already had deliverability issues before migrating. The principle is always the same:

Slow is smooth. Smooth is fast. Warmup is earned, not forced.

8.10. Monitor as you go and adjust instantly

Warmup is not a straight line. You’re watching, daily:

Soft bounces
Throttling
Spam placement by provider
Complaint spikes
Weird engagement drops
Any difference between providers

If Outlook gets grumpy?
You slow down Outlook only.

If Gmail throttles?
You pause Gmail’s ramp for a few days and only send to engaged.

If Yahoo starts soft-blocking?
You reduce Yahoo’s volume AND tighten the engagement criteria.

This is why a warmup plan is not a spreadsheet but a living process.

9. Send positive signals first

Warmup isn’t just about how much you send - it’s really about what you send.

Inbox providers aren’t sitting there with calculators counting your volume. They’re watching behaviour. They’re asking:

Do people really want these emails?
Are people opening and clicking and engaging?
Do they reply?
Do they move the email from spam to inbox?
Do they do anything that signals “I value this”?

During warmup, every single email you send is essentially teaching Gmail, Outlook, Yahoo, and everyone else: “This sender is legitimate. These emails are wanted. This behaviour is normal.”

Which means the content you choose for your warmup phase can either accelerate trust… or signal “red flag, suspicious sender”.

This is why you cannot send rubbish during warmup.

Send targeted content, not mass blast

This is not the moment for:

“Let’s hit everyone and see what happens”
“We need to drive revenue this week”
“It’s fine, they’ll open eventually”

You want extremely predictable, high-quality engagement. That means sending to people who have recently interacted, not everyone who’s ever been on your list since 2016.

Warmup is the opposite of a “spray and pray”. It’s a “send carefully and observe everything”.

Do not try to re-engage with disengaged subscribers in the first 12 months of moving provider.

Lead with value, not selling

Sales-heavy emails historically perform worse during warmup because:

Lower engagement (especially opens)
Higher delete-without-opening
Higher complaints
More spam button taps

All of these are negative signals, and negative signals slow the warmup down and damage deliverability.

Warmup email content should be:

Helpful
Educational
Expected
Relevant
Familiar in tone and purpose
Predictably engaging

Examples:

A genuinely useful resource
A valuable checklist or something that solves a problem
A quick win tip
A soft update people actually care about
A high-value editorial piece
A “here’s something you asked for/waited for” email

Positive signals = faster warmup

Inbox providers watch aggregate behaviour. They are looking for proof that you are who you say you are.

They love seeing:

High open rates from engaged users
Clicks that look natural (not bot-like)
Low complaint rates
Low bounce rates
Stable sending patterns
Humans actually interacting with the content

Every positive signal = more trust.

More trust = your warmup can progress faster.

This is why content strategy is part of migration strategy.

A simple rule for content during warmup

If the email does not create predictable, positive engagement, do not send it during warmup.

Save the harder-hitting sales messages until your IPs and domain are stable and inbox providers have finished their sceptical “who are you and why are you in my house?” phase.

Warmup is not about driving revenue!! Warmup is about protecting revenue.

And the best way to do that is by sending content that gently, consistently tells inbox providers that your emails are wanted and trusted.

10. How to explain all of this to leadership

This is the “send this to your boss” bit.

When you talk to leadership, you do not need to drag them through every IP pool. You need them to understand risk, trade-offs, and support.

Here’s a structure you can use:

10.1. The core messages

“Switching ESPs isn’t just like swapping CRMs or project tools. To inbox providers, it looks like a brand new sender has turned up and started emailing our customers.

If we rush the migration, we risk being treated like spammers, which can take months to fix and directly impacts revenue.”

10.2. The risks (in business language)

If we “lift and shift” with no warmup and no audit, we risk:

Revenue loss from emails landing in spam instead of inboxes
Increased complaints and unsubscribes
Brand damage when important messages never arrive
Engineering and support fire-drills to “fix email” later
Slower recovery because inbox providers remember bad behaviour

10.3. The ask

What you need from leadership:

Time: A phased migration (often up to 3 months of warmup and parallel running, depending on scale) rather than a hard cutover date.
Resourcing:
→ Someone to own deliverability and warmup
→ Technical resource for DNS and integrations
Budget:
→ For deliverability audit(s)
→ For an email validator
→ For external support if you don’t have in-house expertise

And then the reassurance:

“This isn’t about being overly cautious. This is the industry-standard way to protect deliverability and revenue during an ESP switch.”

11. So what do you actually do next?

If you’re planning (or already mid-way through) an ESP migration, here is your immediate action list:

Stop promising dates
→ Do not commit to a hard “we’ll be fully migrated by X” without a warmup plan.
Run a deliverability audit on your current platform
→ You need to know whether you’re migrating a healthy programme or dragging existing issues with you.
Get inbox-provider breakdown of your list
→ Run your data through an email validator to tag Gmail vs Outlook vs others.
Sketch a provider-based warmup plan
→ Starting with small, engaged volumes per provider and ramping gradually.
Map all your current automations and must-have sends
→ Decide in which phase each one moves.
Book time with your new ESP’s deliverability team
→ Ask specifically about IP pools, onboarding paths, and recommended warmup schedules.
Educate your leadership team
→ Use this blog as your briefing deck: switching ESPs safely is a deliverability project, not a settings change.

12. Need a custom warmup plan (or someone to sanity check your migration strategy)?

If you’re reading this and thinking:

“We’ve already switched and everything has tanked”, or
“Our leadership want this done in three weeks and I need backup”

…then this is exactly the situation I built my services for.

I offer:

Custom warmup plans - based on your domain history, volumes, and audience mix
Deliverability audits - pre- and post-migration, so you know what’s really going on
Migration support - helping you plan a phased cutover that doesn’t blow up your email strategy

Get in touch here to chat about your needs

And if you’re already seeing open and click drops after a switch, read (and share): Why Your Opens and Clicks Have Dropped After Switching Email Platforms - it walks through the metric side of this in more detail and will calm a lot of panic.

Like this blog? You'll love RE:markable