Let me describe the scenario, and then see what you may say is likely to be the cause:
We've got a page written in PHP on our gentoo server. The page is basically a POST form and emails the details to a client. When we access this from our office and complete the form, the customer receives the email. When the customer does this from their office, with the same form and the same details, they do not receive this. The mail processor on the server is postfix and the call is the basic mail call in PHP, nothing fancy.
At first we thought there might be some random firewall issue but given that the same form is used by both parties, and that the mail processor is the same, I don't realistically see that this would be the case, particularly as the IP address for the server is going to be the same as the originator (clearly the same goes for spam filtering or whitelisted filtering). The issuer email address is the same too, in case you were wondering.
As ever, your thoughts and ideas gratefully received.