Emails does not send

Hello community,i have a [b]big problem[/b] with the automatic Emails (sry for my bad english).If an user sign up a new account, he get´s no validation email.I wait over 20 minutes an resend, but nothing happened.An Erroer appears: 550 relay not permittedMy hoster is hosteurope.I tried various options.if ich turned smtp on & off, i tried gmx, i installed both versions of million dollar script.My Last attempt, it was to change the php files, unfortunately without success.i tried to solve the problem ober 10 hour. If there someone, who can help? I´m desperate. :( pic enclosed.greets bigbobAttachments: