An ESP can hold subscriber lists, send campaigns and maintain a suppression list of addresses that must not receive future sends. Feature coverage differs between providers. (Google email sender guidelines ↗)
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Following a newsletter through a provider change
Illustrative migration checklist: the old platform sends from newsletter.publisher.example and stores suppressions; the new platform may use a different sending setup and list state. Before sending, the publisher maps who controls subscriber records, suppression and message headers, then tests a delivered message.
ESP describes the service, not the audience owner
When changing providers, test an unsubscribed address as well as an active subscriber. Moving the active list and email design without preserving suppression records can cause the new system to send to someone who already opted out. Confirm the actual delivered sender identity too.
Inventory what would move with you
Before changing providers, identify the lists, suppression records, sending domains, signup forms and automations your publication uses. Ask which can be transferred and verify the most important reader flows after migration. A successful list import does not prove that forms or unsubscribe requests still work.