Is this a new problem? If so try sending mail to yourself or to another via your webmail interface.
If its a new set up, maybe your ISP requires you to have authentication turned on?
Or if you have a DSL / ADSL connection, but arent using the email service that the DSL ISP provides, then you will have to probably but the outgoing mail as the DSL or connection provider and the incoming mail as the name of your mail server, ie my email address is for example
[email protected], and I am connected to the internet with DSL.com's service. My outgoing mail server would be something like: smpt.dsl.com and my incoming mail would be something like pop3.fastconnect.com
Maybe that might help