I have a membership based website, and I use Gmail as an smtp server for users to email other users. When one user wants to email another user, they simply email from my website using the form, and the form then sends the message through gmail. The reply-to address contains the senders email. What I see happening, at least judging from the gmail box, is that some recipients respond to the smtp gmail address instead of the reply-to address. How does this happen, and does anybody know of a way to avoid this from happening in the future?