Check your error logs to see what messages are indicated there, if the timeout or 503 is occurring on Roblox's end with their server(s) giving that response, then the issue would be on Roblox's end.
FYI, Argentina has been on the blacklist for quite a while now, about 3 years. Existing accounts are eligible for review for an exception as the error message directly states. Would you like it reviewed for an exception.
Can you create that folder again and let it sit and see what happens.
When in round cube make sure delivery status notification is on and try sending an email, to see if a status message comes through.
Upon further checking, it is a SSL problem since your SSL is only covering your domain itself and no subdomains. I would either recommend changing the set mail server to just be the domain itself or the server hostname.