Every newsletter platform promises seamless imports. Upload a CSV, connect your old ESP, wait five minutes, and you’re done. Except you’re not.
Import tools handle the basics—email addresses, names, subscription dates—but they quietly drop or mangle data that matters. If you’re moving from one platform to another, you need to know what won’t survive the trip before you click “Start Import.”
Custom fields disappear or get renamed
Most platforms let you store custom data: subscriber location, referral source, purchase history, survey responses. When you import to a new platform, these fields rarely map automatically.
Beehiiv, for example, supports custom fields, but you need to create them before importing your CSV. If your old platform called a field “signup_source” and your new one expects “source,” the data lands in limbo or gets ignored entirely.
MailerLite handles this better than most—it auto-creates custom fields during import if it detects columns that don’t match standard fields—but it still won’t infer meaning. If you tagged subscribers as “paid” or “free” in your old system, you’ll need to manually map those to groups or segments after import.
Tags and segments don’t transfer consistently
Tags are metadata. Segments are dynamic queries. Most platforms treat them differently, and import tools don’t convert between the two.
If you’re moving from ConvertKit (which uses tags heavily) to a platform like Brevo (which leans on lists and attributes), your tags might import as plain text in a custom field—or not at all. You’ll need to rebuild segments manually using the new platform’s logic.
Even when platforms do support tags, they often cap how many can attach to a single subscriber during import. ConvertKit allows unlimited tags per contact, but some smaller platforms limit you to 10 or 20. If you’ve been tagging aggressively, the import will silently truncate.
Engagement history gets wiped
Open rates, click rates, and email engagement scores don’t port over. Your new platform starts everyone at zero.
This matters more than it sounds. Most ESPs use engagement history to throttle sending for new accounts. If you import 10,000 subscribers and immediately send to all of them, your new platform sees 10,000 cold contacts with no prior opens. That triggers spam filters and damages your sender reputation faster than a gradual ramp-up would.
Postmark and other transactional-focused platforms don’t track opens by default (they’re built for receipts and password resets, not marketing), so if you’re moving to a marketing ESP, you’re starting engagement tracking from scratch no matter what.
The fix: after importing, send only to your most engaged segment first—people who opened in the last 30 days—then gradually expand. This warms up your sender reputation on the new platform without triggering alarms.
Unsubscribes and suppression lists need manual handling
Unsubscribes should transfer, but they often don’t. Some platforms export unsubscribed emails in a separate CSV. Others include them in the main export with a status column. If you miss that column during import, you’ll re-subscribe people who opted out.
Suppression lists—emails that hard-bounced or marked you as spam—are even trickier. Most platforms won’t let you import suppressed addresses at all (by design), but they also won’t automatically suppress them on your new account unless you manually upload a suppression file.
If you skip this step, your first send on the new platform will attempt delivery to addresses that bounced months ago. That kills your sender score immediately.
What to do before you import
Export everything from your old platform: subscribers, unsubscribes, suppressions, custom fields, tags. Save separate CSVs for each.
Check your new platform’s import documentation—not the marketing page, the actual help docs—and map your fields manually. Create custom fields and tags before importing, so the data has somewhere to land.
Import in stages. Start with a small segment (100–500 subscribers) and verify everything looks right: names, fields, tags, suppression status. Only then import the full list.
And plan to rebuild segments. Even if tags import cleanly, your new platform’s segmentation logic probably works differently. Budget an hour to recreate your most important segments by hand.
Switching platforms isn’t one click. If you’re moving soon, factor in a half-day for data cleanup—not five minutes.
If you’re running into platform-specific import issues—or you’ve found a tool that handles this better than most—reply and let me know. I’ll cover it here.
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