When starting Outlook, I was getting a cert error that said this. What I did to fix it:
1. Removed all the certs that it would let me from the Exchange server. I had to leave the default there so that I did not break SMTP.
2. Created a new cert with the following in there for the SANs
Don’t forget the last SAN of internaldomain.local alone. Imported this certtificate, and enabled it for IIS and SMTP, replacing the old certificate. I then deleted the old one, and all was well again.