SMTP Recipient Verification
XG lacks the SG feature of being able to query the backend AD or Mail Server to verify the destination recipient.
By default when XG is acting as a MTA it will pass on a e-mail even if it's addressed to a user that does not exist. SG used to be able to poll Active Directory to ensure the address was valid (or you could disable this).
This feature should be ported into XG as soon as possible.