Are your outgoing mail server account, username and password correct? (check them, and remember that passwords are case-sensitive)
Is the SMTP authentication working correctly at your mail server? (ask your ISP to check)
Make sure you are connecting to your mail server's 'authenticated mail' port 587 and not the ordinary 'unauthenticated' port 25. (ask your ISP to check for you)
Mail servers only run spam filters such as Spamhaus PBL on port 25, so if you find you are being blocked by the PBL when you try to send mail to your mail server that means you are not communicating with the mail server on the 'authenticated' port 587 but you're still on port 25. This means your 'SMTP Authentication' is not working correctly. (ask your ISP to check for you)
In summary, if you are being blocked by the PBL when you try to send mail from your PC or Mac to your mail server that means you are sending on port 25, which means that even though you think you have SMTP Authentication switched on, your SMTP Authentication is not working correctly. This can often be something as simple as a bad password, see "Things to check" above.