• What type of mail server do you use for SMTP? If its Exchange and you use the mail relay functionaity you need to ensure that you allow the IP address of the server where the mail is being sent from is allow to connect and send via the mail relay admin GUI, there are two places you need to add the IP.

    Then also check that you dont have some sort of antivirus issue going on where it blocks mass mail worms, just to be on the safe side as I know from past experiences we had issues with McAfee stopping the mail services of SQL from sending and needed to be over-ridden by en ePO policy change.