We've seen a few issues recently where members of DGs (Distribution Groups) in mixed-mode Exchange 200x and 5.5 seem to randomly and mysteriously disappear. We thought we'd share one known root-cause and also show you how to prevent the problem while doing your migration, if for whatever reason you are still running Exchange 5.5 =). One easy way to prevent this problem (and a few others) is to ALWAYS use the Exchange 2003 post SP1 cross-site mailbox migration wizard if you ever have to move mailboxes in a mixed-mode environment.
With that said, typically the problem occurs when you use a directory export/import to move mailboxes between Exchange 5.5 sites. For example, if you have two Exchange 5.5 sites Site S (Source) and Site D (Destination) and you would like to move all the mailboxes in Site S to Site D you might perform the following steps. (When we refer to a DL we mean a Distribution List on the Exchange 5.5 side and when we refer to DG we mean a Distribution Group on the Active Directory side)
The problem occurs after step 4. The following key points should help you understand why.
In this situation, since the DLs' USN-Changed isn't incremented in the 5.5 directory when you delete the mailboxes, the corresponding DG (Distribution Group) membership in the AD isn't updated.
If you later make changes that increment the USN-Changed on the DL, the entire object (including the earlier deletions) is replicated to the AD side and so some members seem to randomly disappear from the DG's members list. While replication in Exchange 5.5 is object-based and replication in the AD is attribute-based, Exchange 5.5 to AD replication is still object-based (think lowest common denominator). The solution to this problem is to force the USN-Changed on the DL to increment after performing the deletions in step 3. A DL's USN-changed increments under the following scenarios:
Again, a DL's USN-Changed does not increment when you use the Exchange 5.5 administrator program to delete a mailbox from the DLs membership. To force the USN-Changed value on the DL to increment you need to make a 'dummy' change that falls under a, b or c after step 3. Our new steps to ensure we avoid the problem would therefore be:
Everything should work fine and dandy at this point and you needn't worry about 'ghosts' modifying your DGs!
- Jasper Kuria and William Yang