For a customer I ran into a strange problem. While doing a cross forest mailbox migration I noticed that mails were queuing up and were not delivered. It took some troubleshooting untill I found the solution. First I noticed that newly created users received email for several domains without any problems. Migrated users didn’t receive any external email.
I opened the user’s properties with ADSIEDIT and I noticed the old email addresses on the targetaddress field. I decided to delete the values and re-register those email addresses using the Exchange console. The user concerned received external email immediately .
Because it’s a cross forest migration we are using ADMT for the user migration. It’s possible to exclude this value and doing so will prohibit to these old values being migrated to the new domain.