I’m attending a training course this week (getting back up to speed with Exchange using the Core Solutions of Microsoft Exchange Server 2013 course) and I found that one of the labs wasn’t working. I probably missed a step somewhere and I wanted to verify a dynamic distribution group based on a custom attribute.
Back in the days when I was hands-on with Exchange, there was an Exchange-aware version of Active Directory Users and Computers (ADUC) to work with, but that disappeared in 2007. These days, the solution is to open ADUC, enable the Advanced view and then look at the Attribute Editor. It took some searching but I found that what Exchange refers to as Custom Attribute 1-15 are actually known by the LDAP names of extensionAttribute1 (to 15) in Active Directory.
Once I’d edited extensionAttribute1 in the user object(s) to add the value the dynamic distribution group was looking for, they were picked up and mail flowed as expected.
One last point: whilst Exchange 2013 doesn’t allow a preview the contents of dynamic distribution groups within the Exchange Admin Center it is possible inside the Exchange Management Shell.
[Update: Custom attributes can also be set in EAC - under more options for mailbox properties - as well as in EMS, where the attribute is known as CustomAttribute1)]