Is it possible to have an ‘OWA only’ user mailbox so it cannot be added to a mail client?

Generally, mail clients use protocols like MAPI, POP, IMAP, and Exchange ActiveSync (Mobile). You can turn off these protocols in the Exchange Admin Center (EAC) for Exchange 2019 by following these steps:

• Open the Exchange Admin Center (EAC) and navigate to Recipients > Mailboxes.

• In the list of mailboxes, find and select the mailbox you want to modify.

• Click Edit to open the mailbox properties.

• In the mailbox properties page, go to the Mailbox features section.

• In the Email Connectivity section, you will see the protocols and can disable your desired protocols.

• Click Save to apply the changes.

You can also achieve this by using the Set-CasMailbox cmdlet to disable other client protocols like MAPI, POP3, and IMAP for the user mailbox. Here’s how you can do it:

• Disable MAPI:

  Set-CasMailbox -Identity "UserMailbox" -MAPIEnabled $false

• Disable POP3:

  Set-CasMailbox -Identity "UserMailbox" -PopEnabled $false

• Disable IMAP:

  Set-CasMailbox -Identity "UserMailbox" -ImapEnabled $false

• Disable ActiveSync:

  Set-CasMailbox -Identity "UserMailbox" -ActiveSyncEnabled $false