When a large number of contacts are imported to an audience, we review them through our abuse-prevention system to ensure that the audience can maintain its reputation over time.
The first email campaign you send to this newly updated audience will send slowly so that our system has time to verify the imported addresses. This process doesn't affect existing, already-verified contacts.
Verification is a well-oiled machine, so even new subscribed contacts will receive emails in a reasonable time frame.
In the event that an import cannot be verified, we'll cancel the remaining sends to the newly imported contacts and we may ask that you undo your import.