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.
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:
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?”
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:
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.
From your boss’s perspective, not much changes:
So why is it so fragile?
Because under the hood, a lot shifts at once:
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:
Most ESPs have different pools for:
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:
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.
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:
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.
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:
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:
Let’s spell this out in non email friendly language.
When you change ESPs, inbox providers see changes in:
Any one of those can trigger extra scrutiny. Changing all of them at once? That’s a full-scale investigation.
Filters ask:
If they don’t like the answers, they will:
That’s why the migration strategy is not about convenience; it’s about managing the amount of change inboxes see at any one time.
A warmup is not:
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:
New IPs and “cold” domains are suspicious by default. Warmup is your “prove we’re legitimate” phase.
This is the bit I really want you and your boss to understand.
Most “DIY” warmups go like this:
Sounds fine on paper. But no, absolutely not.
Chunk 1 might be:
Chunk 2 might be:
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:
So you can’t just split your list randomly and expect a coherent warmup.
The right way to do this:
The actual numbers depend on:
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.
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:
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:
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:
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.
A quick sidebar here, because the deliverability audit step is where most teams realise they either:
If that’s you, you’ve got two options:
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.
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.
Let’s walk through an idealised version. You’ll adapt it for your scale, but the phases are the same.
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:
8.2. Inventory everything in your current ESP
Literally everything:
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:
They need to understand:
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:
This is the plumbing phase.
8.5. Configure the account properly
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:
Get this in writing where possible. It saves pain later.
8.7. Migrate data and workflows carefully
You might be tempted to import historic reports too, but that’s optional. What matters most during migration is that your current system works.
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.
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:
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”.
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:
Run your audience through an email validation tool so you know:
You cannot warm effectively without this picture.
You need clean cohorts for:
Each provider learns at its own pace and they do not share reputation signals.
This is why random, linear list-splitting doesn’t work.
Start with tiny volumes. Think microscopic, not “comfortable”.
For example:
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:
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.
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:
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.
This is not the moment for:
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.
Sales-heavy emails historically perform worse during warmup because:
All of these are negative signals, and negative signals slow the warmup down and damage deliverability.
Warmup email content should be:
Examples:
Inbox providers watch aggregate behaviour. They are looking for proof that you are who you say you are.
They love seeing:
Every positive signal = more trust.
More trust = your warmup can progress faster.
This is why content strategy is part of migration strategy.
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.
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:
“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.”
If we “lift and shift” with no warmup and no audit, we risk:
What you need from leadership:
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.”
If you’re planning (or already mid-way through) an ESP migration, here is your immediate action list:
If you’re reading this and thinking:
…then this is exactly the situation I built my services for.
I offer:
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.
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.