Hey @Sando04 - is this for your Optusnet email address? If so, it may be that the connection/server you're using overseas is blocking certain ports.
Are you able to log in to your Optusnet email via our Webmail Portal instead?
I don't believe you can send mail when you are connected to another network outside Australia. Even with authentication ticked for your outgoing server I think it would fail. I guess Optus webmail or changing the out going server to the network you are connected with should work.
Edit: or should I say add another outgoing mail server as I believe the iPhone will allow you to have more than one outgoing server. Try authentication on your outgoing mail server first and see what that does.