Sooner or later in your relationship with an ESP, you’ll have to force the moment to its crisis. Now, whether the crisis is real or imagined is where most brands go wrong. 

ESP migration can be such a double-edged sword.

Over the years, it has earned a notorious reputation, thanks to horror stories about ballooning costs, misplaced or wiped-out data, broken automations, and so on. 

It’s no surprise, then, that brands approach it with anxiety. And yes, it’s a tricky job. 

That’s exactly why the first step isn’t planning the move. It’s asking a harder question: Do you actually need to migrate, or are you reacting to second-hand panic?

So before we talk about migrating to Sailthru, let’s confront the harder question. 

When is it time to change your ESP?

Your email list has grown 

Most email providers charge based on subscriber count, meaning your monthly bill can skyrocket as your list grows. If the cost per subscriber begins to outpace the revenue they generate, you’re no longer running a profitable channel.

Beyond cost, there is a technical ceiling. Smaller platforms often struggle with deliverability at high volumes; if an ESP lacks the infrastructure to send thousands of emails simultaneously, your messages may be delayed or flagged as spam. To avoid a crisis, you should audit your provider’s pricing and reliability against your projected growth at the earliest. 

Your deliverability has tanked  

Top-tier providers invest heavily in infrastructure, engineering talent, and direct ISP relationships to ensure their traffic is trusted and prioritized. Not all platforms operate at that level.

If your technical foundation is solid, your list hygiene is tight, and deliverability still lags, it may be your ESP. One practical way to validate this?

Split a campaign and send a portion through a different provider. 

Be especially cautious when blaming deliverability. Deliverability is deeply technical, and often misunderstood. Many teams lack the expertise to properly diagnose issues, which makes it easy to point fingers at the ESP. Don’t confuse technical complexity with platform incompetence.

Your analytics leaves you guessing

You need a platform that provides behavioral trends and revenue attribution, allowing you to spot engagement decay or deliverability issues before they tank your ROI. If you find yourself manually exporting data to spreadsheets just to understand the performance of your campaigns, your current provider is likely a bottleneck to your business intelligence.

The “divorce” becomes necessary when your ESP limits your ability to experiment. 

You’re feeling the need for better integration 

To achieve a single view of the customer, your data must flow across your entire ecosystem. The gap between a vendor’s on-paper integration and real-world utility lies in latency and depth. 

Because as far as claims are concerned, they’re neither here nor there. 

So, the goal is to find a vendor that acts as a central nervous system, capable of triggering the right message on the right channel at the right time. 

This isn’t an exhaustive checklist. But these are the four core signals that it may be time to switch your ESP. That said, recognizing when not to migrate is just as important.

When to NOT migrate 

Here are a few scenarios where you should pause before deciding to switch:

• Many brands switch because they want advanced automation, only to realize their current ESP already had those tools, but the team simply hadn’t been trained to use them.
  
• If you’re switching solely to save money, factor in the cost of team retraining, recreating dozens of automation workflows, and the potential revenue dip during the transition.
  
• Low engagement, high bounce rates, stale lists, weak segmentation, etc. won’t be fixed by a new provider. In such a case, you clearly have a strategy problem.
  
• If an agency or partner is pushing an ESP switch without clear, business-specific technical reasons, they may be prioritizing kickbacks over your stability.

Assuming you have the right reasons for making the switch, we’ll talk about how you can migrate to Sailthru from your current ESP. 

Migrating to Sailthru for email marketing, and more

Before you sign the contract 

The contract comes first. You’ll likely be facing some tension here, but you can reduce its impact, provided you proceed wisely. Here are a few things to bear in mind:

• Go into negotiations with a clear sense of how many data sources you plan to connect, how rich your user profiles will be, and how many domains you’ll be sending from.
  
• If a feature is pivotal to your migration rationale, get clarity on when exactly it’s arriving. Building your plan around something that’s still months away can be risky.
  
• Don’t forget that you will be paying for two ESPs simultaneously for sometime. Make sure to set your budget accordingly. 

Sailthru has a quote-based, customized pricing model. Profile limits, data richness thresholds, number of domains, and number of data sources can all trigger incremental costs. 

Build the right team 

Migration cuts across every team in your organization: marketing, IT, product, legal, e-commerce, analytics, and more. The biggest mistake brands make is underestimating this and bringing people in mid-stream, when decisions have already been made and there’s costly catching up to do. But you can’t just knock a team together. A few things to consider: 

• Identify your decision-makers. At a minimum, you need a senior business owner and a technical counterpart who are jointly accountable and visibly aligned.
  
• This is non-negotiable: Find a project manager who can orchestrate multiple teams, track hundreds of dependencies, and keep everything moving.
  
• Migrations are rarely quick. In one engagement, moving from Salesforce Marketing Cloud to Sailthru took us close to six months. So, build a buffer into your timeline.
  
• Try to make your ESP mirror your org’s structure. In other words, the implementation team should have counterparts for each of your key roles. That person-to-person pairing keeps decisions moving and avoids single points of confusion.

Willy-nilly, the team will hit roadblocks daily. So you’ll need a clear hierarchy so those roadblocks get resolved quickly. 

Audit and cleanse your data 

Moving bad data into Sailthru is the fastest way to get blacklisted or to spend your first months firefighting. So, before you export a single contact:

• Remove hard bounces, spam traps, and consistently disengaged subscribers. Then map every automated journey you currently run. Screenshot the logic, because you will likely need to rebuild it from scratch in Sailthru’s Lifecycle Optimizer.
  
• Record your current open rates, click rates, and bounce rates as a performance baseline. You’ll need these to verify whether Sailthru is actually delivering better results. 

Create a comprehensive migration checklist. List every element that needs to move: contact lists, suppression lists, segments, templates, automations, data points, integrations, and workflows. And don’t just document what you currently use, document what you intend to build. The migration is also the moment to fix gaps in your data architecture that have accumulated over time. Validate that everything flowing into Sailthru is clean, complete, and pulling from the correct source of record.

Validate assumptions during data discovery sessions. Broad statements like “we have transaction data” can mean different things to technical and marketing teams. Clarify the specifics: what data fields are available, at what level of granularity, where the data originates, how frequently it refreshes, how much historical data exists, and what safeguards are in place if data syncs fail.

Technical setup 

• Authorize Sailthru to send emails on your behalf. Accordingly, update your DNS records with Sailthru-specific values provided in your account settings. Set up a link domain so that all tracked links in your emails use your own domain, not Sailthru’s.
  
• Next, swap your current ESP’s tracking script with the Sailthru v1.js tag. Make sure you are tracking pageview and purchase events immediately.
  
• Sailthru uses Profiles and Vars, or variables. Decide which data points (Vars) from your old system are essential.
  
• Next, start importing your contacts. For bulk uploads, use the native CSV tool. Import your suppression list first to ensure you don’t accidentally email an unsubscribed contact. If you have larger datasets, developers should use the POST /user API.
  
• Sailthru uses Zephyr, a proprietary templating language. If you are switching from Klaviyo (Liquid) or Mailchimp (Merge Tags), you will need to translate your logic.
  
• Recreate your automated journeys using Lifecycle Optimizer (LO).

Also audit every integration your old ESP was running. Your CRM, data warehouse, e-commerce platform, and analytics tools all need to be reconnected. Map these integrations before migration, confirm their compatibility with Sailthru, and test all connections early. 

IP and domain warming 

You cannot send your full volume on Day 1. You have to warm your new IP and domain to build trust with ISPs. With that in mind, consider the following best practices: 

• Identify your most engaging content historically and your most engaged users. Start by sending that content to those users at low volume. Why? This maximizes the positive engagement signals Sailthru’s new sending infrastructure sends to inbox providers. Then build gradually, doubling the volume every few days.
  
• Monitor your sender reputation daily using tools like Google Postmaster. Track hard and soft bounces, spam complaint rates, open rates, and click rates. If results look poor, pause. Fix the issue before resuming. Don’t push forward blindly hoping things improve.
  
• Use a hybrid approach. Keep sending from your old ESP while warming Sailthru. So, if something misfires, you’re still safe. It also means you can take the time to do things properly without putting revenue at risk. Never compress your timeline so tightly that your old ESP contract ends weeks after your Sailthru warm-up begins.

Gmail behaves differently from other ISPs and should be planned for independently in your warm-up schedule. One common mistake is tripling the send volume too quickly, which can tank deliverability specifically there and require a full Gmail re-warm to recover from. Therefore, understand your database composition and plan your Gmail ramp accordingly.

Managing the transition period 

There will be weeks, possibly months, when you are straddling both platforms. Therefore, you’ll need to prepare for it. Here are a few considerations to that end: 

• Your suppression list must stay synchronized across both ESPs in real-time. If a contact opts out through a campaign sent from your old ESP, that opt-out must be reflected in Sailthru before your next send, and vice-versa.
  
• Manage frequency carefully. If campaigns are running on both platforms simultaneously, a customer could receive emails from each on the same day.
  
• If you have journeys that trigger based on email clicks or opens, a click on your old ESP won’t be visible to Sailthru. Build the bridging logic where necessary, or sequence your migration so that dependent campaigns move together.
  
• Track performance across both platforms in parallel so you can validate that migrated campaigns are performing comparably. Any significant drop post-migration may point to a configuration issue worth investigating before you migrate more campaigns.

Tying the loose ends 

• A common failure pattern is that project managers and technical teams lead the migration, then hand it off to marketing staff who’ve never touched the new tool. Give your team sandbox access to Sailthru early. Encourage actual use, not just watching training videos, but building things, breaking things, and asking the right (and wrong) questions.
  
• Start with a pilot group if possible, and identify internal champions who can support their colleagues during the transition.
  
• Keep critical automations running on your old ESP until Sailthru is fully configured, tested, and the team is genuinely comfortable.

“A smooth ESP migration isn’t about speed, it’s about strategy. The goal is to protect your deliverability, maintain a seamless customer journey, and come out the other side stronger than before.”

— Sara Kappler, founder & CEO, Centric Squared

We recommend you keep your old ESP active for at least 30 days after Sailthru is fully live. Use this time to run out any long-running drip campaigns so users don’t receive duplicates or experience gaps in their journey. Do a final suppression export from your old ESP before you close the account so that anyone who opted out mid-migration is carried into Sailthru. 

Migrations can be tiresome. Need help?

Migrations are long, and they wear people down. The people working on yours are carrying this project alongside everything else in their lives. Recognize that. Communicate the business value of the shift right from the beginning and keep reinforcing it throughout. 

But you don’t have to shoulder it alone.

Our team has executed migrations across 50+ ESPs with a 99% delivery accuracy rate. If you’re evaluating a move, book a no-obligation call with one of our experts today!