The Complete ESP Migration Checklist: Switching Platforms Without Losing Revenue

An ESP migration is one of the highest-risk operations in email marketing. Done poorly, it can damage your deliverability for months, create gaps in your automation coverage, and cause revenue disruption that takes a full quarter to recover from. Done well, it unlocks capabilities that accelerate your programme for years.

The difference between a migration that goes smoothly and one that turns into a crisis almost always comes down to preparation. Brands that skip the pre-migration audit, underestimate the warm-up period, or treat the cutover as a single event rather than a managed transition are the ones that end up with deliverability problems and lost revenue.

This guide is the complete migration framework — from pre-migration audit through to 30-day post-migration monitoring. Use it as a checklist alongside whatever detailed ESP-specific documentation is available for your particular platforms.

Phase 1 — Pre-Migration Audit (4-6 Weeks Before Cutover)

Before you touch your new ESP, you need a complete inventory of what you are moving. Most brands discover during this phase that they have more complexity than they realised — dormant flows that are still sending, segments built on deprecated properties, integrations that were set up by someone who has since left the business.

Document All Existing Flows

Pull a full list of every automated flow in your current ESP. For each flow, document:

Flow name and trigger condition
Number of emails in the flow and their sequence
Current performance metrics (open rate, click rate, revenue generated)
Any filters or conditional splits within the flow
Dependencies on specific custom properties or integration data

This documentation serves two purposes: it ensures nothing is missed during rebuilding, and it gives you a baseline to compare against post-migration performance.

Document All Active Segments

Export a list of every segment in your current ESP. Note the definition logic for each segment (especially any that use custom properties or complex conditions) and identify which segments are used by flows vs. campaigns vs. both. Some segments may be redundant or outdated — migration is an opportunity to rationalise your segment architecture.

List every active signup form connected to your ESP, including embedded forms, popups, checkout opt-in integrations, and any landing page forms. Note where each form is embedded or hosted and what list or segment it feeds.

Map All Active Integrations

Document every integration connected to your current ESP — e-commerce platform, loyalty programme, reviews platform, CRM, analytics tools. For each integration, note what data flows in each direction and whether there is a native integration available for your destination ESP or whether a custom connection will be required.

Phase 2 — Data Migration Planning (3-4 Weeks Before Cutover)

With your audit complete, you can build a detailed data migration plan. The core questions are: what data needs to move, in what format, and in what order?

Export Your Contact Data

Export your full contact list from the current ESP, including all custom property values, subscription status flags, and historical engagement data (where exportable). Most ESPs allow CSV export of contact data — the challenge is that field names and data types may not match your destination ESP.

For each custom property, confirm whether the destination ESP has an equivalent field and map the old property name to the new one. Document this field mapping before any import begins.

Export Your Suppression List First

Your suppression list — unsubscribed contacts, hard bounces, marked-as-spam contacts — must be the first thing you import into your new ESP. This is the single most common mistake in ESP migrations: brands switch ESPs and accidentally send to people who previously unsubscribed because the suppression data was not transferred.

Sending to a suppressed contact is a compliance violation (GDPR, CAN-SPAM) and will generate spam complaints that immediately damage your new sender reputation before you have even warmed up your domain.

Plan Your Segment Rebuild Strategy

Decide which segments you will rebuild by re-creating the definition logic in the new ESP versus which you will handle by importing a static list from the current ESP export. Dynamic segments (those defined by behaviour rules) should always be rebuilt with native logic in the new ESP. Static segments (snapshots of a contact list at a point in time) can be imported as tags or custom properties.

Phase 3 — Build and Test in the New ESP (3-4 Weeks Before Cutover)

With your data plan in place, begin building in the new ESP. Run the old and new ESPs in parallel during this phase — do not touch the old ESP’s configuration until you are ready to cut over.

Rebuild All Flows

Recreate every flow from your audit documentation. For each flow, rebuild the trigger logic, email content, timing delays, and conditional splits. Do not just copy content — use the migration as an opportunity to review and update flows that may be outdated.

Set Up DNS Authentication for Your New Sending Domain

Your new ESP will require DNS records to be added to your domain — specifically SPF, DKIM, and DMARC records for authentication. These records confirm to ISPs that the new ESP is authorised to send on behalf of your domain.

If you are using a custom sending domain (which you should be), ensure the DNS records are updated before any test sends go out from the new platform. Authentication failures are a major deliverability risk.

Create a Comprehensive QA Test Protocol

Before going live, every flow should be tested end-to-end. Send yourself through each flow trigger, confirm that conditional logic is working correctly, check rendering across major email clients (Litmus or Email on Acid are the standard tools), and verify that all links, UTM parameters, and personalisation tags are functioning correctly.

Phase 4 — Warm-Up Plan (2-4 Weeks Before Cutover)

Even if you are migrating an existing list to a new ESP, you are establishing a new sending domain or IP address. ISPs have no sending history for your new configuration, which means they will treat your initial sends with heightened scrutiny.

The Warm-Up Sequence

Begin warm-up sends 2-4 weeks before your planned cutover. Send to your most engaged subscribers first — those who have opened or clicked in the last 30 days. Start with small volumes (1,000-2,000 per day) and ramp up gradually over two to three weeks.

As your engagement metrics on the new platform build a positive reputation, progressively expand your send volume to less recently engaged segments.

Do not attempt to send your full list during the warm-up phase. A large spike in send volume from a new sending configuration is a strong spam signal to ISPs.

Phase 5 — Cutover Day Sequence

Cutover day is the moment you switch from sending on your old ESP to sending on your new ESP. This should be treated as a planned event with a clear sequence.

The Cutover Steps in Order

First, confirm that all flows on the new ESP are active and all forms are redirecting to the new ESP. Second, export a final suppression list from the old ESP and import it into the new one (to capture any additions in the period since your initial migration). Third, pause all active flows on the old ESP. Fourth, confirm the new ESP is the active sending platform by running a small test send. Fifth, officially declare the cutover complete and begin monitoring.

Plan for a Parallel Period

Some brands choose to run a brief parallel period (typically 3-7 days) where both ESPs are technically active but the old ESP is set to only send transactional emails while the new ESP handles all marketing sends. This provides a safety net if critical issues are discovered post-cutover.

Phase 6 — 30-Day Post-Migration Monitoring

The migration is not complete on cutover day. The 30-day period following the migration is critical for catching and correcting issues before they become serious problems.

Daily Monitoring (Days 1-7)

Check deliverability metrics — bounce rates, spam complaint rates, and unsubscribe rates — daily during the first week. Any unusual spikes should be investigated immediately. Review flow performance to confirm triggers are firing correctly. Check that form submissions are being captured in the new ESP.

Weekly Review (Days 7-30)

Compare key performance metrics week-over-week and against your pre-migration baseline. It is normal to see some performance variance in the first 30 days as your sender reputation builds on the new platform. However, significant and sustained drops in open rate or click rate warrant investigation — either of your content, your warm-up strategy, or your authentication configuration.

Review the integration data flows weekly to confirm that customer data, event triggers, and custom properties are syncing correctly between your e-commerce platform and the new ESP.

ESP migrations require the right expertise to execute without disruption. Excelohunt has managed multiple complex migrations — including mid-size e-commerce brands moving from Mailchimp and Klaviyo’s competitors — and we bring a proven framework that protects revenue through every phase of the transition.

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The Complete ESP Migration Checklist: Switching Platforms Without Losing Revenue