While you might not have changed configuration of the Moodle, doesn't mean there haven't been changes to the SMTP server. So before chasing a rabbit down a hole thinking it's moodle, understand what the error is saying:
The initial connection failed (HELO command - reference:
The client sends this command to the SMTP server to identify itself and initiate the SMTP conversation. The domain name or IP address of the SMTP client is usually sent as an argument together with the command (e.g. “HELO client.example.com”). If a domain name is used as an argument with the HELO command, it must be a fully qualified domain name (also called FQDN).
From your moodle server's command line, see if port 25 is opened to your moodle - might have to install nmap to scan that port:
nmap -P0 -p 25 mailserverIPorFQDN
If the port is open, install a text based MTA (like alpine) and configure it to relay mail to the mail server using whatever credentials setup in Moodle.
If neither works, then assume it's on the mail server side and contact mail server administrator.
Note: many mail servers now do not use port 25.
'spirit of sharing', Ken