Retain keeps an "item store flag" to ensure that no item gets left behind.
With Exchange and O365 Holds and the Recoverable Items folder can be used for retention compliance. With On-Premise Exchange a journaling mailbox can be used but it is not recommended.
Gmail, by default, does not have a retention service.
GroupWise, on the other hand, has its own built-in feature called "Retention Services" that prevents items from being emptied from the mailbox until they have been successfully archived.
The following sections explain Retain's support of the GroupWise Retention Services, followed by a discussion of how Retain ensures that all items get archived in all other email systems.