Scenario: Users are complaining that they sporadically miss email communications when the emails were sent to a specific distribution groups. Upon further investigation of the distribution groups, the recipients that did not receive the email were members of other distribution groups that were nested two,three, and/or four times of that distribution group that was originally sent to.
Troubleshooting: After pulling the message tracking log against all exchange servers, we noticed that the distribution groups were not expanding by seeing a 0 recipient count:
get-transportserver | Get-messagetrackinglog -subject “Testing” -resultsize unlimited -eventID Expand | Select RecipientCount,RelatedRecipientAddress
After investigating the distribution groups that were not expanding, we found that these distribution groups were mail enabled security groups AND they were set to Global and not Universal. Apparently these groups were changed back to Global after the mail enabled security groups were created so they could be members of other global distribution groups. Thus we were experiencing ‘Black Hole’ distribution groups.
Solution: After the user changed all the groups from Global to Universal, mail flow returned to normal.