Your VPS host (which one it is, please let @adamjedgar know 
) is blocking port 25 for devices external to their network. BTW, many other VPS hosts do this too, see MX records, Thunderbird, Postfix, Dovecot.
Before you poke the beast and change too many things on your box, please look around to see how others have addressed this issue. Also see Free smarthost / mail relay to work around port 25 block by VPS host