Sending mail with @hotmail.com in from field is causing errors

[Ben Trinder]

I run a bunch of sites (IIS4) and an Exchange 5.5SP1 Server.
When sending a web-form through ASP with the Exchange Server as the SMTP host, everything works 100% reliably until the from field contains a HotMail address.

The follow error is sent to the mail administrator, and a similar Event Log entry occurs
************************************************** *****
A mail message was not sent due to a protocol error.

555 spakt@hotmail.com is not in your domain
The message that caused this notification was:


To: <internet.feedback@compaq.com>
From: <spakt@hotmail.com>
Subject:
************************************************** *****
There should be a subject on the mail, and the error is produced within a few seconds of SUBMIT being pressed on the web form.
Every other mail domain I have tried works (even ones that the Exchange Server knows nothing about, but are valid).

Has anyone had a similar problem, or can anyone suggest something to try... I have tried everything I can think of (I`d rather not change the from field, otherwise the recipient of the form won`t be able to hit reply)

thanks in advance...