I traced the code and it seems that EHLO wasn’t even reached. The SMTP server is connected but the SSL handshake seems to be missing, so the server quits and Discourse didn’t get any response, thus the error.
So, I suspected that port 465 support seems to have a bug (it defaults to SSL, but I don’t think Discourse recognizes that) so probably the SSL connection is never established.
Switching to port 25 seems to at least get rid of the error, although I am not receiving any sent email yet…
EDIT: I can confirm that port 25 works properly (as should 587 but unfortunately my smtp server doesn’t expose 587).
I didn’t get any emails before was due to the fact that the Discourse server IP address was temporarily blacklisted by the smtp server because of too many failed attempts.