Zend Framework 1.6.2 Manual

In case you want to send different e-mails through different connections, you can also pass the transport object directly to send() without a prior call to setDefaultTransport(). The passed object will override the default transport for the actual send() request:

send($tr1);
$mail->send($tr2);
$mail->send();  // use default again

Note: Additional transports

Additional transports can be written by implementing Zend_Mail_Transport_Interface.