ield. 
<tt>Rcpt</tt> is a Lua table with one entry for each recipient e-mail
address, or a string
in case there is just one recipient. 
The contents of the message are given by a <em>simple</em> 
<a href="http://lua-users.org/wiki/FiltersSourcesAndSinks">LTN12</a> 
<tt>source</tt>. Several arguments are optional:
</p>
<ul>
<li> <tt>user</tt>, <tt>password</tt>: User and password for
authentication. The function will attempt LOGIN and PLAIN authentication
methods if supported by the server (both are unsafe);