smtp;550 relay not permitted

We have win2000-Sp3, IIS, SMTP & SQL 2000 installed. There are as many as 5 sites hosted on this server. On one site there is an ASP form which accepts users input & send it to an email address.

User see no error. But in the backend after sendingit, that mail gets placed in the Drop folder. Earlier it was getting bounced in the Bad mail folder with error

The error is

I’ve checked the SMTP relay settings. I’ve even enabled the relay for anonymous. But nothing worked.

Any idea?