what are you using for the email server and port? who is your host.
in many instances, the server and port can be left blank, or set to localhost and port 25, but the setting to use depends on the host.
X-Mailer is an optional setting. It is the client used to send the email, in your case it would be "PHP [version]" where [version] is the version of PHP running on your host server. but it can be left blank.