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 ...
Updated Jul 01, 2019
Version 2.0
Blog Post
Also, the "Active Directory Clean-up Wizard" adds the old X500 address to the merged user object.
(For those who have not used the ADCW, ADCW merges the Active Directory Connector-created disabled user accounts or contacts that correspond to an Exchange 5.5 mailbox with the AD user that owns the Ex5.5 mailbox.)
(For those who have not used the ADCW, ADCW merges the Active Directory Connector-created disabled user accounts or contacts that correspond to an Exchange 5.5 mailbox with the AD user that owns the Ex5.5 mailbox.)