It may be a long shot, but have you tried different ways of entering the mail server hostname, trying both with and without the https prefix in the Emailer:

https://smtp.office365.com

Of course you'll have to use the proper hostnames, the above are just examples.

Isn't https and SSL synonymous ?

Besides, the error message talks about a version number issue, not a protocol issue.

To a human, yes :-)

I've googled the error message, and it seems it might have to do with the protocol version having changed from PROTOCOL_SSLv3 to PROTOCOL_TLS in Python 3.5.

I tried to change the encryption in the Emailer to TLS (instead of SSL), but that only gave me another error:

SMTPDataError: Could not complete request. Message: (554, b'5.2.0 STOREDRV.Submission.Exception:SendAsDeniedException.MapiExceptionSendAsDenied; Failed to process message due to a permanent exception with message Cannot submit message. 0.35250:5B2C0000, .... [Hostname=VI1P190MB0542.EURP190.PROD.OUTLOOK.COM]')

Digging deeper.

Ok. my digging did uncover the cause. It is indeed the encryption, that now has to be TLS, not SSL.

It seems that this was changed in Python 3.5, and since 2020.0 doesn't support any older versions (except for 2.7 if installed), TLS is the new default.

My last obstacle was my weary butter fingers, that misspelled "relay" as "reply" in the credentials setup. After that was corrected, the email was sent correctly.