One of my customers has just finished migrating their user mailboxes from Exchange 2003 to Exchange 2010. After the migration of users my customer experienced a weird NDR (Non-Deliverable Report). This NDR was generated when an Exchange 2010 mailbox user emailed a mail enabled distribution groups containing a mail enabled contact object in Active Directory.
This problem only occured for a select few distribution groups. The NDR received was:
#< #5.6.1 smtp;554 5.6.1 Body type not supported by Remote Host> #SMTP#
The Exchange server generating the NDR was one of the old Exchange 2003 servers. Now why would Exchange 2010 be delivering this email to Exchange 2003? This gave it away. Straight away I looked to see if there was an Expansion Server configured for the distribution group. Yes there was - it was pointing at one of the old Exchange 2003 servers. I simply removed the Exchange 2003 server as an expansion server for the distribution group.
To view if your group has an expansion server configured, go to the properties of the distribution group in Exchange Management Console and click the Advanced tab.
To view if any of your other distribution groups have expansion servers configured for Exchange 2003, use the following powershell command in Exchange Management Shell.
Get-DistributionGroup | fl Name, ExpansionServer
What is an Expansion Server?
Expansion servers route messages that are sent to a single distribution list or group for each of the recipient objects in that list or group. When a user sends a message to a group, the Exchange server that is acting as the expansion server expands the group to its individual members. This expansion permits members of the distribution list or group to receive the message. An expansion server also resolves the names of all recipients in the distribution list or group, and then determines the most efficient path for routing the message.
You configure which hub transport server or Exchange 2000/2003 server you wish to use as your expansion server on the distribution group.
If you do not designate a specific server as the expansion server that expands a message that is sent to a group, the first server that the message is submitted to expands the group, and then sends the message to all of the destination servers.
NOTE: There is a drawback to setting a specific server as the expansion server for a group. If that server is down, no members of the distribution group receive the message.