Mail Source Agent does not synchronize some of the mailboxes that are added to a mailbox collection via Distribution Group if source Exchange server is Exchange 2010 DAG.
To search for the members of a Distribution Group added to mailbox collection, the Mail Source Agent uses homeMTA attribute of the Administrator mailbox specified for the source Exchange server. It synchronizes only those group members which have the same value of the homeMTA attribute that the Administrator mailbox has.
If source Exchange server is Exchange 2010 DAG the mailbox database can be moved from one DAG member to another. homeMTA attribute is populated when a mailbox is created and never changes even if the mailbox database moves to another server. Hence, if some mailboxes have been initially created on one of the DAG members which homeMTA is not equal to the one of the Administrator mailbox selected for mailbox synchronization and then the mailbox database moved to another DAG member which homeMTA corresponds to the one of the Administrator mailbox, their homeMTA attribute will not be changed with the mailbox database move and the Mail Source Agent will not recognize such mailboxes as the members of the Distribution Group added to mailbox collection.
The only workaround for the issue is adding mailbox to mailbox collection either explicitly or via Container (OU or LDAP query). Distribution groups cannot be used for adding mailboxes to mailbox collections if the source server is Exchange 2010 DAG.