While Firewalls and Antivirus products are designed to protect you, they also can impair your emailing ability. A good way to test this is to temporarily disable your firewall or antivirus software. If you are able to send email after disabling the security software, then you may need to adjust your firewall/antivirus settings or download an update for the software. For more information on temporarily disabling your protection software, contact the manufacturer of the software.
NOTE: Make sure you re-enable your security software after testing!