When sending email (SMTP), if you see this error it means that you need to provide SMTP authentication in order to send email outside your domain.
Make sure you’ve set both the mailman.SmtpUsername and SmtpPassword properties.
550 5.7.1 Unable to relay for email@example.com
Failed when sending RCPT TO:<somebody123 @gmx.at>
Does that mean that even if I use Windows Integrated Security I always have to provide a username and password as soon as I want to send a mail outside my domain?
Set both the SmtpUsername and SmtpPassword properties to "default", and Chilkat will use the current logged-on credentials (assuming your SMTP server supports NTLM authentication).