Originally Posted by bluegrass
I did try it, but still there is an error. I guess the problem is that I configured my SMTP server IP (on the mail client) the public IP of the Anti-Virus/Anti-Spam, hardware where the mail server is connected. But when I tried the same mail client on another PC which is also connected in parallel with the mail server I don't encounter a problem at everything works out fine.
If it's working on another computer, then it must be somekind of misconfiguration in your email client.
For example, you configure Outlook for SMTP-AUTH like this: http://mail.cs.uiuc.edu/relay/outlook-config.html