I have XP with service pack 2 upgradation few days back + Panda Titanium+ Sygate Firewall platform.
Since almost one month I am unable to sent email to persons more than four or five through outlook express. Earlier it was fine and I can send mails to any no. of people.
Most of the AV’s and Firewalls disable sending more then 4 e-mails to prevent spamming, and mass mailing worms. Check your AV settings in that case. I am not familiar with the software hence cant help there.