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