A couple of users (out of 150) are experiencing an issue where they send out emails or message invitations, but they are never received by the recipients. When i go into Exchange System Monitor, and into the Message History for the email, it shows the status just stops after "SMTP: Message Submitted to Categorizer". the message just seems to disappear after this. It's only for a few users, and only on occasional emails.
details about our network, Exchange 2003, McAfee Virus Scanning on the computers (but email scanning is disabled), and we have an IronPort device doing email spam filtering, but i don't even see the emails showing in the message tracking on the IronPort, so it isn't even making it that far.