are there any known issues with the smtp server?
what email client are you using? outlook express? ms outlook? thunderbird?
check the address of the outgoing mail server that you are using in the account settings.
it should look something like eg..
the middle part being the supplier of the email account.
lastly and a common is (providing account settings are ok) is with antivirus software or firewalls.
check your settings to ensure you are able to connect to your out going mail server.