A user who is no longer working for the primary organization, but where their new email is referenced in the active directory system for business communication purposes. The user is reporting that they are receiving spam quarantine notifications and other messages from the messaging gateway at the new email address on the sub domain.
The cause is that the messaging gateway device is seeing the new email address on the Domain Controller via Directory Integration authentication. This is, effectively, working as designed.
Remove the user information from active directory, or customize the directory integration query such that it doesn't reference that user when the query is run.
Messaging gateway is configured for user defined quarantine and is setup for Directory Integration.