The email address that is to be used in the 'Reply-To'
field when sending the email using SMTP. The format of
this object is an e-mail address which conforms to RFC822
and RFC1123 'addr-spec' syntax.
Some examples are
raj@helpme.com,
bob@service.com,
mtom@abc.caview.ca.us.
A set operation on this object with a invalid format will
result in an error.