Set the reply address for the contact form to user's mail address
Isn't there a way to set the reply addresses to the ones of registered users and why isn't it done by default?


When authenticated users send a message using the system contact form, the email address assigned to the sent message is the one set for their user account; I just tested it on my Drupal 7 test site, and I can confirm it works that way.

The issue you are seeing is probably caused by the fact you are relaying the emails through Gmail.
If you need to add any header to the email sent from Drupal to resolve this problem, then you can implement hook_mail_alter() in a custom module.