Email Problems

Email systems typically use standard port numbers, 25 for SMTP, 143 for IMAP, 110 for POP3. Setting filters for these ports will usually help to discover the cause of problems with email.
Virus Detection and Control
Anti virus software manufacturers offer updates services. Armed with the information on new threats it is often possible to build suitable filters to detect viruses. For example many analyzers allow you to specify a text pattern so a virus contained in a message containing a known text string could be detected. Analysis of the capture will show the source and destination of the packets.
Firewalls
Firewalls need to be checked for outgoing and incoming traffic. You will have to define a set of filters for traffic in both directions. Should the firewall begin to let unauthorized traffic through you need to be able to detect it.