Out of the blue one of our users will not be able to send emails to a MailUser in Outlook, but it will work with OWA
The error is IMCEAEX-_o=NT5_ou=00000000000000000000000000000000_cn=73495F221CFA124085D6606DD6101CFF@sendingdomain.com
If I delete the contact completely from Outlook suggested contacts, and any other contact list, the error still remains
If I create a new Outlook profile, I still get the error.
I tried the solution in this post, but it did not help, the legacyExchangeDN is populated and correct.
Another user in the post reported that a corrupt OAB on the Exchange server might be an issue if there are multiple CAS servers/DAG - but I only have a single server with all roles installed
Anyone any ideas?