MimeIn MBC stops when receiving MDNs with following error in message log:
INFO 2009-09-11 14:45:03.88 EmailReceiveReport EDITorg EMAIL_Rec_Activity Email received
INFO 2009-09-11 14:45:03.89 SyncPointEvent1 EDITorg MIMEIN_Seq_Activity User defined syncpoint
ERROR 2009-09-11 14:45:03.90 MimeInMDNReceived EDITorg MIMEIN_Seq_Activity The MIME MDN refers an unknown message
ERROR 2009-09-11 14:45:03.90 HierchMsg EDITorg MIMEIN_Seq_Activity An exception was caught in the MBC

It has been observed that Original-Message-ID stands missing in the MDNs generated by MS Outlook (2003 SP3) clients. As a result exception generated with an error that 'MDN refers an unknown message'.
Switch to MS Outlook (2007 SP2) it generated a beautiful MDN including the Original-Message-ID at the end of the message.