cannot add Exchange Mailbox to Outlook for MAC and native Mail app

Hi,

We have a strange problem for a few days. The client has Exchange Server 2019 on-premises. The problem is suddenly email stopped delivering to the mailbox which was added to Outlook for MAC or the native macOS mail app. No problem with mailboxes configured on iPhone or Outlook for Windows.

Today I tried to troubleshoot the problem on another MAC computer. I cannot add an Exchange mailbox to Outlook for MAC or Mail app. "couldn't verify username or password". Password is fine since I can log in to OWA. I was also able to configure the same mailbox on an iPhone or Outlook for MAC.

I think for some reason MAC is more strict or a DNS resolution with the certificate is the problem. I ran the MS Analyzer Connect test and the output is fine.

Any idea why we cannot add an Exchange mailbox in MAC? IMAP is also not working.

Thanks, Edy