Generally said, there are three different ways of fighting spam mails.
They are utilized on the following levels:
To ensure that these protections are working the right way, spam has to be
detected as spam. This is why, spammer-lists are used, which contain addresses and domains.
Using these criterias, a certain amount of spam mails can be filtered, theoretically.
In reality the limitations of this system can be seen very fast:
About 80 percent of all spammers are using faked headers and change them very often.
The more secure way is hunting for keyword in the mail-body.
- Gateway Level:
Gateway Filters work, as the name says, as a filter for a whole mail-domain.
It's often a native application or a firewall add-on.
- Server Level:
Server filters join an increasing popularity. Similar to gateway filters
server-side filters work on the mail server. See
anti-relaying for more information.
- Client Level:
But also the end user can utilize anti-spam tools on their local
workstations. They are often used as additional local security instance for
the providers anti-spam mechanisms.