Webmail not working - Regular pop/smtp does

[bin_asc@]

I`m trying to use the webmail to send emails, or even reply. I get this error :

There was an error sending your message: unable to add recipient [[email protected]]: Invalid response code received from server

I think I went over each possible thread related on the forum regarding this issue, and I have yet to find any solution.
One of the problems is that I can`t even find the error code in the logs. Help ...