Webmasters using OS Commerce and formmail scripts on our servers might experience problems setting up the posting of customer forms due to the following restriction in our mailserver:
Either the ‘From:’ or the ‘To:’ field of the form must contain an e-mail address hosted on our server.
By default, these forms use the webmaster’s e-mail address in the ‘From:’ field (which is usually hosted on a different server) and that’s the reason messages are rejected.

In this case please correct the ‘From:’ or the ‘To:’ field. Check the PHP/Perl section of the FAQ for a custom example of a working formmail script.