how do SPAM filters know which messages are bad?

Question

i know that sometimes legit messages are treated as spam, but generally how do spam filters know legit messages from the bogus ones?

Answer 1

Spam messages are tracked in two ways.



1. They track how many messages come from the destination IP and check it against a database of trusted nonspam mass vendors such as newsletters and flag those not included)



2. Through user intervention, if you mark something as spam all future emails from that address are automatically filtered.

Answer 2

In addition to Mike's answer, a private network (school, business) will filter spam by blocking any message that contains URLs, photos, attachments and/or anything that isn't text.

Answer 3

If the can has been opened already.