Pass a pointer to a zero terminated string as parameter. This will be used to
specify the authentication address (identity) of a submitted message that is
being relayed to another server.
This optional parameter allows co-operating agents in a trusted environment to
communicate the authentication of individual messages and should only be used
by the application program, using libcurl, if the application is itself a mail
server acting in such an environment. If the application is operating as such
and the AUTH address is not known or is invalid, then an empty string should
be used for this parameter.
Unlike CURLOPT_MAIL_FROM(3) and CURLOPT_MAIL_RCPT(3), the address
should not be specified within a pair of angled brackets (<>). However, if an
empty string is used then a pair of brackets will be sent by libcurl as
required by RFC2554.