I remember seeing a document regarding Lotus/MSF co-existence...... http://www-912.ibm.com/s_dir/slkbase.NSF/0cf528a492c9b4a88625680b0002037f/26a1c5ded443408b862565c2007cfb76?OpenDocument
The Domino for AS/400 Message Transfer Agent (MTA) hooks into MSF
and registers for all SMTP/MIME address types. Domino for AS/400
shares the same domain as the OS/400 system. MSF will first call
the OS/400 SMTP address resolution snap-in. If the OS/400 snap-in
is not able to locate a user in the SDD/system alias table, then
the message is marked so that it is passed to the Domino MTA.