It's a critical oversight imo, that the Zend_Mail classes do not permit the specification of a timeout value when using network-connect protocols.

As example reference, consider

Zend_Mail_Protocol_Imap on line 88:

$this->_socket = @fsockopen($host, $port);

should be replaced with a call that includes a timeout variable that can be set in the transport config.

$errno = "";
$errstr = "";
$this->_socket = @fsockopen($host, $port, $errno, $errstr, $this->THE_TIMEOUT );

In addition, the $errno and $errstr variables should be leveraged to provide better error reporting, eg:

$this->_socket = @fsockopen($host, $port, $errno, $errstr, 10 );
if (!$this->_socket) {
            /**
             * @see Zend_Mail_Protocol_Exception
             */
            require_once 'Zend/Mail/Protocol/Exception.php';
            throw new Zend_Mail_Protocol_Exception( 'cannot connect to host : ' .$errstr );
        }

This is easy to implement, and may provide more salient error reporting than 'cannot connect to host'.