This picture is of quote from the blog post about migrating to a new ESP correctly. Read the post to get your email deliverability checklist.

Ready to Migrate ESPs? Here’s Your Email Deliverability Checklist

Are you migrating to a new email service provider (ESP)? Follow our email deliverability checklist to ensure success.
Article Outline

Marketers switch email service providers (ESPs) for various reasons. Your current sender may have a poor sending reputation, lack scalability, cost too much for what you get, and/or be missing features you’d like to have. If you’re ready to make a change, we’re offering an email deliverability checklist to help you succeed on your fresh start.

As a deliverability analyst, the number one reason I see customers switching, or wanting to switch, to marketing automation is they have experienced poor deliverability with their previous stand-alone ESP platform. My job is to ensure that with their move to marketing automation, they will get the most messages delivered to inboxes through a secure infrastructure and best practices.

But it’s not quite as simple as it may seem. You can’t just switch ESPs at a moment’s notice. ESP migration involves a great amount of thoughtful planning and preparation. There are many moving parts in deliverability that need to be taken into account, and any false moves or missing steps could spell disaster for your email marketing success.

This article focuses exclusively on the deliverability components you should be aware of in order to be successful in your migration to a new ESP.

Suppression Lists

At Act-On, we classify suppression lists into:

  • opt outs
  • spam complaints
  • hard bounces
  • inactive soft bounces (soft bounces that exceed a certain threshold and are considered undeliverable)

Understanding the differences between hard and soft bounces can be confusing if you’re not in the trenches with your email deliverability team. Traditionally, a “hard bounce” indicates a permanent reason an email cannot be delivered. The usual reasons are an invalid email or an invalid domain.

“Soft bounces” have been defined as temporary failures due to an unavailable server, a full mailbox, or an oversize message. While those causes still hold true in theory, they no longer represent the top reasons behind today’s soft bounces, which are mostly caused by internet service provider (ISP) blocks. In most cases the blocks are temporary and can often be resolved by identifying the root causes, then following email best practices and ISP guidelines. Read our post Understanding and Reducing Soft Bounces to better understand how to identify and fix soft bounces.

Your email service provider might classify them differently, or might not classify them at all. Whatever the case, these are contacts that must be uploaded into your new account and suppressed, as they either don’t want to receive email from you or are simply undeliverable addresses. Not properly suppressing these contacts will start you off on the wrong foot and cause your reputation with the new ESP (and your customers and prospects) to quickly suffer.

Engagement Data

If you decide to use a dedicated IP with your new ESP, being able to identify and migrate over your engaged recipients will help establish a good IP reputation. By sending to your most engaged contacts from the new IP during the initial warm-up period, you can show ISPs that you are sending to people who want your email, and thus may have less of a chance of being blocked or labeled as a poor sender.

“From” Domains

When you bring your old sending domains with you, be aware you are carrying over the good – or bad – sender reputation you’ve established over the years. Despite the fact that changing domains is a spammer’s practice and highly discouraged, a new “From” domain does allow you to start fresh with a clean slate and gives you a second chance to be a good sender moving forward.

Data Hygiene

Though not a prerequisite for an ESP migration, practicing good data hygiene can go a long way toward ensuring your onboarding is as smooth as possible. A list cleanse is recommended every six months or so to help keep your list as clean as possible. Most ESPs have their own database of bad and risky contacts to suppress against globally, so just because you weren’t running into issues with the previous ESP doesn’t mean the issues weren’t there.

Embracing list maintenance best practices ensures you get optimal deliverability and performance on your campaigns. Here are some tips for measuring how clean your email list is.


Email authentication is an integral part of your infrastructure for mass mailing. Make sure Domain Name System validation (DKIM) and Sender Policy Framework (SPF) are properly set up at the corresponding domains with the new ESP, as your old setups will not migrate over.


To ensure successful delivery of test and live messages from the new platform to your internal servers, you may need to whitelist your new sending IP(s) and domain(s) to prevent potential hiccups.

Whitelisting is the opposite of blacklisting. It lets your email service provider know that you are a willing recipient of the emails sent from that IP address.

Check out our post on common terms for the email marketer, parts one and two.


Last but not least, while we understand you expect a lot more from the new partnership, transient deliverability problems can occur during the warm-up process. You may experience temporary blocks by ISPs and dips in your engagement rates. This is normal for any new IPs and/or domains you are trying to build up.

To help speed up the process and minimize the time needed for the transition, we recommend fully migrating to the new ESP from the get-go and refraining from using both the old and new ESPs simultaneously, a behavior that exhibits similarity to snowshoe spamming (a spammer’s strategy for pushing spam over multiple domains and IP addresses – spreading out the weight – to avoid filters).


And remember, a new ESP shouldn’t replace email marketing best practices such as A/B testing your subject lines and other components of your email, segmenting your lists, and being thoughtful about your sending schedules. More tips on improving your deliverability and open rates can be found on our post The Blame Game : Open Rates vs Deliverability.

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