The consequences of an email being wrongly classified can range from a minor inconvenience to a major catastrophe. If a spam email is marked as ham, then the recipient will only spend a few seconds removing it from their inbox. If an unimportant ham email is marked as spam, it may be no great problem. However, marking an important email as spam could be embarrassing at the least, and could well result in serious consequences, such as financial loss.

Consequently, it is worthwhile making a backup of emails marked as spam before purging them. Emails compress well, so it may be possible to keep this backup on local disk storage rather than tape or other removable media. When it becomes apparent that an email has gone missing, the archive of spam can be searched and the email retrieved.

Due to the potential cost of an incorrect classification, users should be encouraged to review any spam they have received on a regular basis. Any false positives should be brought...