If you are having trouble sending mail, but you can receive mail fine, this implies that your computer can't make an outgoing connection to whatever outgoing mail server you have specified in your mail client.

You need to set your outgoing mail server to that of your ISP (e.g. if you are with Telstra Bigpond, you would need to use mail.bigpond.com). This is because most ISPs are actually blocking outgoing mail requests to anything other than their own mail servers.

A quick call or email to your ISP will be enough for you to get this information from them.