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.