You could uncomment the lines further below to get a full SMTP log where you can see exactly what your server is complaining about. However, I think your server isn’t complaining, I think it simply cannot be reached through the network.
https://github.com/salesagility/SuiteCRM/blob/master/include/SugarPHPMailer.php#L487-L492
I would make those messages FATAL instead of DEBUG, to make sure you see them easily in your log.
Look for DNS issues, firewall issues, try some pings, try reaching your SMTP server from the SuiteCRM server via Telnet.