Hello and welcome to the forum 🙂
The error message is clear: There is a communication issue between the SMTP client (eg. the r3 server) and the SMTP server on port 587.
Telnet is a good thing to try. To troubleshoot further: Do you know that the same SMTP settings work on another SMTP client on the same server? You could test this with something like Thunderbird.
If the settings are correct and the server has no network issues, you should be able to use another email client to send via SMTP on the same system. If it does not work, we know that either the settings are incorrect or there is a network issue.
