Since it did not come from that persons email client or computer it is really all up to what they log about the user sending the email from the form. I would find it hard to believe that they would not have the IP address of the user logged. Since they require many pieces of information. Please keep in mind that if the person is not a complete idiot they may have used a proxy and anonymous email service to hide their identity. Does the site use a relay? (Does not allow users to directly send email to each other) If it does then it complicates things further. In a nutshell, would their info be in the header? Most likely no. But they should be able to give you the IP address of the person that sent the message. Whether or not it will be their IP address or a proxy IP is unknown until you can actually get the IP from them if they hand it over. Don't focus on the mail header. It sounds like it will be useless. Ask to see their database records that stores the IP address related to a posting.

Since it did not come from that persons email client or computer it is really all up to what they log about the user sending the email from the form. I would find it hard to believe that they would not have the IP address of the user logged, since they require many pieces of information. Please keep in mind that if the person is not a complete idiot they may have used a proxy and anonymous email service to hide their identity. Does the site use a relay? (Does not allow users to directly send email to each other) If it does then it complicates things further. In a nutshell, would their info be in the header? Most likely no. But they should be able to give you the IP address of the person that sent the message. Whether or not it will be their IP address or a proxy IP is unknown until you can actually get the IP from them if they hand it over. Don't focus on the mail header. It sounds like it will be useless. Ask to see their database records that stores the IP address related to a posting.