SwiftMailer "Connection could not be established with host"

I've got a problem with SwiftMailer throwing the following error on the client's server: Uncaught exception 'Swift_TransportException' with message 'Connection could not be established with host mail.somedomain.co.uk [Connection refused #111]' in /home/client/public_html/classes/SwiftMailer/classes/Swift/Transport/StreamBuffer.php:266 It works absolutely fine on any other server I've been