The Mailer property returns the value of the X-Mailer
header field in the current message. This is typically used to
identify the application that created the message, however it is not
required that this be specified. If the header field is not present
in the message, this property will return an empty string. Setting
this property will change the value of the X-Mailer header. If the
property is set to an empty string, the header will be removed from
the message.