Can you please share, what Tweak settings did you change exactly?
That's not something we can help with. If cpanel configured your server in a way, it's cpanel which must undo this restriction.If some one doesnt have access to WHM/Cpanel. So how can this issue be fixed.?
If that is disabled by the server admin (for various reasons, incl good ones),make external SMTP connections
Can you switch on debugging and see what output you get?Not using any special characters in the password.
And have disabled it as well from WHM
you switch on debugging