When you switch email service providers (ESPs), your list isn’t the only thing that needs to come with you. Your sender reputation needs to make the move too. Mailbox providers (like Gmail, Yahoo, Live, Comcast, etc.) look at BOTH the “from” address of an email together with the IP address that it came from.

When you switch to a new ESP, your emails are now going to be coming from a new set of IP addresses.  So, you still need to go through a warm-up period when first starting out.

And if you don’t warm it up properly, the consequences can be brutal.

Let me tell you about a real-life case that shows exactly what not to do, and how you can avoid making the same mistake.

One of my clients recently moved to a new ESP. They did what they thought was right: started small, sending campaigns to just 50 to 100 contacts at a time.

They kept that pace for about a month, and everything seemed fine. Open rates looked good. Bounce rates were low. No warning signs.

Then — boom — thinking everything was all clear, they sent a campaign to 19,000 contacts in one go.

Almost immediately, their emails stopped reaching inboxes. Mailbox providers flagged the sudden volume jump as suspicious. Their emails got blocked. And just like that, a full month of campaigns came to a screeching halt.

It took another full month to get the blocks cleared, their reputation restored, and their emails accepted.

Mailbox providers (like Gmail, Outlook, and Yahoo) don’t just watch what you send — they pay attention to how much and how fast your sending volume changes.

Sudden spikes are a red flag.

Think of it from their side: if a sender goes from 50 to 19,000 recipients overnight, it could look like a compromised account or a spammer trying to blast a list before getting caught.

That’s why warming up your IP and domain is so important — it builds trust gradually.

Warming up your IP address and domain is the process of slowly increasing your sending volume to show mailbox providers that you're a trustworthy sender.

Here’s how to do it:

Switching ESPs is a smart move when you're ready to level up your email marketing — but only if you bring your sender reputation along for the ride.

Warm-up isn't just a technical step; it's a trust-building process. Skip it, and you might spend more time cleaning up the mess than enjoying your new platform.

So, if you’re planning a move, plan your warm-up too.

Want a checklist to help you warm up properly? Grab my free Email Warm-Up Guide here.