SMTP sending as recpient

[ScampMD]

I have configured my VB site to use SMTP to send email via my Exchange server (smtp port 25). I created an email account for the site and VB does send the emails, however they are sent with a address of the recipient in the from field. For example, if John@foo.com were to register then the activation email to him would be from John@foo.com. This causes a lot of sites to reject the email.

I've been through the email options and I cannot locate where this setting is. If I log in to the email account directly and send a test message it works fine.

Thanks for any help.