Discourse - SMTP from OVH

One more question: could you please try to execute telnet to your OVH mail server?
I think this may have something to do with authentication expected by my OVH mail server.

I tried this:

    telnet pro2.mail.ovh.net 587

got:

    220 pro2.mail.ovh.net Microsoft ESMTP MAIL Service ready at Sun, 17 Jan 2021 15:50:55 +0100
    EHLO mydomain-assigned-to-my-emailpro.pl
    250-pro2.mail.ovh.net Hello [185.56.211.110]
    250-SIZE 104857600
    250-PIPELINING
    250-DSN
    250-ENHANCEDSTATUSCODES
    250-STARTTLS
    250-**AUTH GSSAPI NTLM**
    250-8BITMIME
    250-BINARYMIME
    250 CHUNKING

This makes me think, that my mail account in OVH has some authentication type which Discourse is unable to understand.
What do you get here when you try telnet to your server? do you get AUTH LOGIN or AUTH PLAIN perhaps?

EDIT: and one more question: do you have authoritative or non-authoritative email pro account? This may also matter I guess. I’ve got non-authoritative as I couldn’t configure my OVH mail in authoritative mode.