The eventlog on our SMTP Server under IIS6 reported the following error very often:

Source: smtpsvc
Category: none
Event ID: 4006
Description: Message delivery to the host ‚xxx.xxx.xxx.xxx‘ failed while delivering to the remote domain ‚mydomainname.ext‘ for the following reason: The connection was dropped by the remote host.
German: Die Nachrichtenübermittlung an den Host „xxx.xxx.xxx.xxx“ ist fehlgeschlagen, während an die Remotedomäne „mydomainname.ext“ übermittelt wurde. Ursache: Die Verbindung wurde vom Remotehost getrennt.

The following page brought me to the solution, even if I did not believe in it:

http://www.dylanbeattie.net/docs/iis6_bare_linefeed.html

In short: don’t have any bare linefeed chars in the message body. Just replace the \n with \r\n and it should work (be careful if you have both \n and \r\n in the body already).