Problem: You have set up the ADC Connector and migrated your users and everything looks good. Then for some reason you need to delete and remigrate the migrated users you will run into a problem. The ADC will try to replicate with the deleted users and not with the new ones.

Reason: The reason for this is that the ADC writes an ADC-Global-Names attribute to the object in Exchange 5.5. This is a unique tag for each user and when you remigrate the users this will not be the same.

Resolution: Open Exchange System  Manager on Exchange 5.5 in raw mode and delete the ADC-Global-Names attribute.

Links:

Google Newsgroups
XADM: How to Remove the ADC Global Names Attribute from Exchange 5.5 Server Recipients
How to roll back a failed migration from Exchange Server 5.5 to Exchange 2000 Server or to Exchange Server 2003