Sophos MailMonitor SMTP

I’m currently trying to implement Sophos MailMonitor SMTP without success. For some reason it is unable to relay mail to my Exchange 5.0 (SP2) NT4 server. The Exchange server is an open relay (one of the reasons for implementing MM) and MM has been configured as open to accept connections from all sources (for troubleshooting purposes). MM has been tried on 2 servers including the Exchange server itself. Different incoming ports have been tried and relayed to port 25 on the Exchange box. Mail is received OK when port forwarded directly through the firewall to the Exchange Server or relayed by any other SMTP software. Sophos tech support suggest that its a problem with the Exchange server but I’m somewhat confused as to why the problem only exists with MM. Any ideas?