Is it possible your provider is interfering in some way? Does the POP3 email account you created for receiving email still work? Does it work if you log into it using a POP3 client? If it’s gmail, you have to explicitly allow POP3.
I don’t know how Discourse might be the source of your problem, especially if it has been working in the past.
If all else fails, you could try taking a backup and restoring it to a new VPN that does not rely on cpanel. Using cpanel is not officially supported by us. In any case, moving to a fresh VPN is always a good idea, especially if you haven’t done it since 2014.