What’s the best way to export/import suppression lists?
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Suppression list transfer might be the single most important step in an ESP migration, and it's also the one most likely to get rushed or handled sloppily. Import your suppressions incorrectly and you'll immediately start emailing people who've unsubscribed or bounced. That's a fast path to complaints and blocklisting.
A suppression list is the combined record of addresses you should never email: hard bounces (addresses that definitively don't exist or are permanently unavailable), unsubscribes (people who opted out), and complaint records (people who marked your mail as spam). Each category matters for a different reason.
To export from your current ESP, look for a suppression, blocklist, or opted-out contacts section in your account settings. Most ESPs let you download this as a CSV with email addresses and suppression reasons. Download everything: hard bounces, unsubscribes, and complaint-unsubscribes as separate files if possible. Keep copies before you do anything else.
Before importing to your new ESP, clean the list. Run it through a validation check to remove any malformed addresses that would cause import errors. Then import each type into the corresponding category in your new ESP. Hard bounces should go in as hard bounces, not just "suppressed," so they get handled with the right retry logic.
If your old ESP can't export suppressions cleanly, ask their support team. Most have a way to pull a complete suppression file. And once you're on the new platform, test with a few known addresses before your first campaign to verify the suppressions are actually working. If you want to also clean and validate your list before migrating, we do that too at Review My Emails.
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