A common cause of mail failing to send is incorrect outgoing (SMTP) server settings. It is an industry wide policy to use the outgoing mail server of the company you are using for your internet access. Failing to do this will often result in 'Error 550' messages
For example, if you wanted to use a Overflow email address whilst working on a Telstra ADSL connection, you would need to use Telstra's SMTP server to send email out through; just like you would use mail.overflow.net.au from a Overflow service.
Below are some quick instructions on how to find these settings in popular mail clients:
In Outlook Express, go to Tools > Accounts. Click on the 'Mail' tab at the top. Click on the account you want to view, pres the 'Properties' button and then select the 'Servers' tab.
In Microsoft Outlook, go to Tools > E-mail Accounts > View or Change Existing Accounts. Select the mail account you wish to view and click on the 'change' button on the right.
In Mac Mail, go to Mail > Preferences. Click on the 'Accounts' button and select your mail account.
Security software such as Antivirus and Firewall packages can cause connection problems to our servers and prevent messages from being sent and received. These errors are normally in the form of pop-up windows which will advise you that the task has failed, rather than a returned email in you inbox. We recommend temporarily disabling your security software to see whether this rectifies the problem.