SwiftMailer not working, connection could not be established [#0] - php

Question

I'm trying to set up a mailer script using Silex and SwiftMailer. However, I haven't managed to get the mail sent so far. When setting swiftmailer.use_spool to false, I don't get errors but don't receive emails either. When instead flushing the spool transport queue like this...
if ($app['mailer.initialized']) {
$app['swiftmailer.spooltransport']
->getSpool()
->flushQueue($app['swiftmailer.transport']);
}
...I get the following exception:
Fatal error: Uncaught exception 'Swift_TransportException' with message 'Connection could not be established with host abc.xyz [ #0]'
I haven't found any documentation for this exception so far. I'm sure all SMTP settings and credentials are valid (I also tried another host, same problem). I can also ping the host and the port I use is open, and php_openssl is enabled.
It looks like it's failing on Swift\Transport\StreamBuffer.php:269 because stream_socket_client returns 0:
$this->_stream = #stream_socket_client($host.':'.$this->_params['port'], $errno, $errstr, $timeout, STREAM_CLIENT_CONNECT, $streamContext);
if (false === $this->_stream) {
throw new Swift_TransportException(
'Connection could not be established with host '.$this->_params['host'].
' ['.$errstr.' #'.$errno.']'
);
}
Update #1
I forgot to have a look at the error log, now I see it says:
PHP Warning: PHP Startup: Unable to load dynamic library '/usr/lib/php5/20121212/php_openssl.dll' - /usr/lib/php5/20121212/php_openssl.dll: cannot open shared object file: No such file or directory in Unknown on line 0
So it seems PHP fails to load the php_openssl extension, although phpinfo() says the module is enabled? Where should it be located, or how should I get it?
Update #2
I believe I may have mistakenly tried to enable the php_openssl.dll extension while it's meant for Windows environments. When I remove it, the error is obviously gone from the error log, but the main issue persists. Back to square one..
What could I be doing wrong?