After completing a migration you may note that replying to emails will cause an NDR. Although sending an email directly to the person works without an NDR. This is most likely caused by the X500 DN changing during the migration. To illustrate this, I will set up a scenario.
You have an Exchange 5.5 environment and it has an Org name of COMPANY and a Site name of DALLAS. You created an Exchange 200x environment with an Org name of COMPANY and the AG is called First Administrative Group.
You migrated the Exchange 5.5 environment using ExMerge or some equivalent method.
If you open one of the email messages it will look normal. If you get the outlook properties of one of the senders of an old message you’ll note that all of the fields are blank except the Display Name. It will have an X500 address in it, which looks like:
/O=COMPANY/OU=DALLAS/CN=RECIPIENTS/CN=TESTUSER. If you open a new message and type in TESTUSER or grab test user from the address book and look at the outlook properties you will see the fields populated and the Display Name will look normal.
The reason for this is that the message is stored with the X500 address at the time it was processed. The migration wizard should adjust this on the fly for you but ExMerge and some other operations will not (you also could find yourself in this type of situation in some single mailbox recovery or other disaster recovery scenarios). The way to fix this is to add an X500 address to the user object in AD that matches their old LegacyDN.