Some of Distribution Groups are synchronized and populated with members but they are not mail enabled. Attempts to resynchronize or mail enable manually do not resolve the issue.
Use the Microsoft utility LDIFDE (or ADSI Edit) to compare the differences between failed and successfully mail enabled distribution lists.
One possible reason could be the attribute "msExchExpansionServerName" being populated.
After using ADSIEdit.msc and changing the value to "any server in the organization" the DL should be processed by DSA and mail enabled.
In some cases populated Exchange attribute "homeMTA" prevents the group from mail-enabling.
The ADSI Edit snap-in can be used to look up and remove this attribute.
If cleaning up resolves the problem but too many objects are affected, the Quest ADCleanup utility or ADModify utility from Microsoft can be used to clean up this attribute for multiple objects.
Once the attribute is "cleared" the group will be processed by the Directory Synchronization Agent and mail-enabled.
See related articles:
Solution SOL15722 - Some Migrated Groups are not Mail-enabled - https://support.quest.com/SUPPORT/index?page=solution&id=SOL15722
Solution SOL9980 - How can I clean up the Service attributes used by DSA (Directory Synchronization Agent) during the matching process? - https://support.quest.com/SUPPORT/index?page=solution&id=SOL9980
The Migration Manager 7.x, 8.0, 8.1 Active Directory Cleanup Utility 1.1 utility can be downloaded from:
Note: since the attribute "msExchExpansionServerName" is being used in source environment and will not be valid in target it makes sense to skip the attribute from synchronization, this will resolve the problem as well.