Delivery has failed to these recipients or distribution lists:
The e-mail system had a problem processing this message. Microsoft Exchange will not try to redeliver this message for you. Please provide the following diagnostic text to your system administrator.
Diagnostic information for administrators:
Generating server: e2003server.domain.com
#< #5.6.1 smtp;554 5.6.1 Body type not supported by Remote Host> #SMTP#
The headers of this message indicate that it was routed through the Exchange 2007 transport servers to Exchange 2003 via the routing group connector and to the Exchange 2003 listed as the expansion server for the distribution list. Most importantly, the NDR generating server was the Exchange 2003 distribution group expansion server.
I immediately began looking into the contact objects for which the NDR was generated. Oddly, I was able to successfully send mail directly to the contact object. I only received an NDR message when sending to a distribution group of which the contact object was a member. Further testing revealed that Exchange 2003 users were able to successfully send messages to the distribution group. The message was delivered to all members including the contact objects in question.
To muddy the waters, NDR messages are not generated for all contact objects in the distribution list. We created several new contacts and added them to a distribution list trying to find the difference between working and non-working contacts. This new test distribution list exhibited the same behavior as others; NDR’ing some contacts while successfully sending the message to others. NDR messages were always generated for the same contacts. At this point I compared the working and non-working contacts but I was unable to find any differences that would cause the problem.
On a hunch, since I was able to send messages to the contacts directly from Exchange 2007, I decided to change the expansion server for the distribution group to an Exchange 2007 transport server. This did the trick and both Exchange 2003 and 2007 users are able to send to all members of the distribution group. While I can’t fully explain why the distribution group does not work when expanded on an Exchange 2003 server, I can accept this simple workaround as the solution to our problems. Hopefully someone more knowledgeable than me can explain this in the comments section.
To resolve this problem set the distribution group expansion server to an Exchange 2007 server. This can be done in the Exchange Management Shell with the following command:
Set-DistributionGroup “Group Display Name” –ExpansionServer:E2007ServerName