SMTP outbound prob with attachments

We are currently having an issue whereas E-mails with attachments are not
sending to certain users. So far I have 3 confirmed cases with 3 seperate
domains. In all three cases IIS is on the receiving side also. This is on
IIS SMTP Windows 2003 SP2. E-mails without attachents to the same folks go
through fine.

Confirmed this to be a problem with IIS as I have tried another 3rd party
SMTP product to relay this mail, and it will go through. The event log error
is 4006 "the connection was dropped by the remote host.". Checking the IIS
SMTP log the last thing logged is "25 BDAT - 4175032+LAST 0 0 4 0 371
SMTP - - - -"

Does anybody have an idea what this issue could be caused by and how to fix
it?