What sort of messages should I not report?

If you have previously had voluntary communication with the sender, then this is not spam and should not be reported.

Other types of messages that should not be submitted are automatically generated messages such as:

Automated Notifications

Undeliverable Mail

Mailing List Probes

Virus Alerts

Challenge Response Auto Replies

It is known that spammers have forged these types of emails, however the purpose of these emails are actually very legitimate. Automated notifications can tell you when a business contact is out of the office, undeliverable email notifications tell you when an email you have sent has not made it to the recipient and why.