I have a linux based mail server (not Microsoft Exchange) at mail.example.com

I host email for multiple domains at this server: example1.com, example2.com, example3.com, etc

I have a mail client running Outlook 2019. When the user adds a new account with their email, e.g.: user@example1.com and then clicks "continue" and then "IMAP/POP", [the incoming and outgoing server fields are blank][1].

Things I have tried to get auto discover working, but don't help