Hi Tim - this is usually when email addresses have bounced a number of times and then get removed from any further emails. This is called the 'Suppressions' list, and it's in place to make sure we don't get blacklisted for spam.
Sometimes emails accounts do get unintentionally added to the list. If you send over the affected addresses to email@example.com, we'll get them reset for you. If it's a different reason, we'll let you know.