- Sounds like an DNS related issue so I would start by validating the Exchange Server DNS settings are correct
- The other action to consider would be to do an AD Health check
- I would also check the local groups to ensurer there are no abandoned/orphaned entries, i.e. they will show as a GUID vs a username
Search, Recover, & Extract Mailboxes, Folders, & Email Items from Offline Exchange Mailbox and Public Folder EDB's and Live Exchange Servers or Import/Migrate direct from Offline EDB to Any Production Exchange Server, even cross version i.e. 2003 --> 2007 --> 2010 --> 2013 --> 2016 --> 2019 --> Exchange Online with Lucid8's DigiScope