Why email performance drops after ESP migration

Switching email platforms is supposed to be seamless. That’s what ESPs tell you. But what they don’t mention is the hidden cost of switching that can destroy your email revenue if you’re not careful. We’ve seen it happen. A brand moved ESPs expecting a smooth transition. Instead, they lost the ability to recognize returning site visitors, and their sending volume dropped for months. Why? Two things happen when brands move from one ESP to another: - They lose cookie data tied to their email subscribers, making it harder to recognize returning customers. - They have to re-warm their domain, meaning lower send volumes and months of waiting for inbox placement to recover. Most brands don’t realize these problems exist until their email performance tanks. There’s a better way. The right ID and deliverability tools can reverse enrich lost cookie data and accelerate domain warming so revenue doesn’t take a hit. If your email provider isn’t preparing you for this, you’re leaving money on the table.

“Wait, so we can just export our list from our old platform, upload it to Klaviyo, and start sending?” A potential client actually asked me that a few days ago. If I had a dollar for every time I’ve heard this, I’d have a separate revenue stream called “Migration Optimism Tax.” Let’s get real: If you think migrating to a new ESP is as simple as exporting a CSV and importing it into Klaviyo, you’re setting yourself up for a mess. Don't assume it's a 5-minute job only to get blindsided by the details. Here’s what gets missed often when brands decide, "let's just do it ourselves," and why it matters: 1. Data Cleanliness Is Non-Negotiable It’s not just about “removing bounces.” You need to hunt down and suppress dead emails from old contests, bots, fakes, and duplicates from messy imports. If you skip this, you’ll tank your sender reputation on Day 1. 2. Segmentation Logic Gets Lost in Translation Your old platform’s logic for “engaged” or “VIP” segments does not just port over. Segments are built on event data that usually doesn’t make the jump. In Klaviyo, you have to rebuild each one manually, mapping rules and sometimes starting from scratch. Test before trusting. Wrong group, wrong message, instant performance drop. 3. Preferences and History Aren’t Automatic Order data syncs from Shopify, but years of custom events, loyalty info, or tags don’t. Same for signup source or list preferences. Miss these, and you lose the context that powers your targeting and flows. Make a mapping doc before you even export. 4. Consent, Tags, and Compliance Require Attention Bulk import makes you responsible for keeping unsubscribes, bounced, and suppressed emails out of future sends. Audit opt-in status, consent dates, compliance tags—anything with legal consequences. If you get this wrong, expect complaints, spam reports, and a headache with compliance teams. 5. QA Is Where Migrations Fall Apart Every flow, segment, and campaign needs to be tested in a real inbox, not just “previewed.” Check dynamic content, make sure you don’t have broken links or “Hi, first_name.” Look at mobile and desktop. Every minute spent here saves you days of chaos later. If you think migration is easy, you’re either new to this or you’ve never checked your numbers after launch. The teams that treat migration as a technical project, not an admin task, are the ones who scale without months of cleanup. If you’ve lived through a messy migration, I want to hear your worst surprise. If you’re planning one, what’s the one thing you’re worried about?

Your inbox performance can fall off a cliff in 48 hours and your ESP dashboard has no clue what went wrong?! That’s the part that messes with marketers the most. I’ve seen teams spend days rewriting subject lines, swapping templates, blaming creative… When the real issue was something boring and invisible. A DNS change. A broken DKIM record. An unsubscribe flow that quietly pushed people to hit “Report spam.” Deliverability drops don’t usually announce themselves. They whisper. The slides below are the exact 48-hour triage order I use before touching copy: 1. Check authentication alignment (SPF, DKIM, DMARC) 2. Look at mailbox breakdowns (Gmail vs Outlook clues) 3. Audit unsubscribe friction (complaints rise fast) 4. Run an inbox placement test before your next big send 5. Freeze volume spikes until you know what changed This is the rule: Fix trust signals first. Then fix content. Want the printable 1-page checklist? Comment TRIAGE and I’ll send it over. Quick question: when performance drops, what do you check first, copy… or infrastructure? #email #emailmarketing

Last week a entrepreneur reached out and said his emails “stopped working.” Open rates dropped. Clicks were down. Messages were landing in Promotions… sometimes Spam. Someone had already told him, “Your strategy might be off.” So he came to me expecting we’d talk messaging. Or subject lines. Or a new angle. We didn’t. Because when performance drops suddenly, I always check trust before tweaking words. His email authentication was the issue. His SPF record was invalid. Not “could be better.” Not “needs optimization.” Broken. Which meant every email he’d sent for months was quietly hurting trust with inbox providers. No copy issue. No content problem. No sudden algorithm shift. Just a technical leak in the foundation. From the sender’s side, nothing looked urgent. Emails were still sending. There was no obvious alarm. From the inbox side, trust was being downgraded a little more with every send. That’s why these problems are so hard to catch. They don’t announce themselves. They show up as performance issues. And people assume the fix is creative. Before rewriting another email, or testing another subject line, it’s worth asking: What if the thing you keep rewriting… isn’t the problem at all? Some problems don’t need better ideas. They need the foundation checked.

Migrations can backfire & cost you time and money. Ryely Newman from Loop Earplugs shared this story during our last Klaviyo Migration Meetup, and the room went quiet. When Loop switched from Klaviyo to Emarsys, everything looked good on paper. Lower cost. More options. One account. A “seamless” migration. Promises were made. Eight months later, CRM performance had fallen out of the sky, revenue and engagement were down 80 percent. 😱 Middleware kept breaking. Data between Shopify and Emarsys went out of sync causing data gaps and broken flows. The team spent more time troubleshooting than sending emails. Consulting hours piled up, adding even more unplanned cost. In the end, Loop went back to and rebuilt on Klaviyo and within months, performance rebounded five-fold year over year. Sometimes moving forward means admitting the last move was wrong. We uncovered a lot more lessons like this at the meetup, I’ll be sharing those next both here and on the KlaviyoGeeks podcast. 👉 If you’ve ever migrated ESPs or CRMs, what’s the one thing that broke or was overlooked that no one warned you about?

One of the biggest migration mistakes I see is when people shut off the old ESP too early. Old emails remain active. Unsubscribe links still work. Complaint and bounce signals still trigger. If you’re not collecting and relaying that data for ~60 days, you risk sending to contacts that should be suppressed in your new system. This can result in having problems with your new system right from the start due to elevated spam complaints, not to mention potential legal compliance issues. This is ALL preventable damage. In this video, I share why suppression overlap matters and tell you about an ESP that makes it super easy to migrate, if you're considering a switch.