Firewalls and antivirus programs on your computer also use this principle to protect end devices from unwanted or dangerous requests. To this end, patterns of harmful information are compared with incoming data. The system then comes to a decision about whether the information will be forwarded or rejected. Known malware is sent into a quarantine area by antivirus programs, where it won’t be able to do any damage.
In addition to proprietary mechanisms and services, there are a number of public providers that function as “spam police”. They maintain spam filters and blocklists. If an email is rejected altogether, the sender will receive an error message from the receiving server with content such as the following:
- Remote host said: 554 Transaction Failed Spam Message not queued.
- Client host [192.0.100.123] blocked using (“suspicious contents”).spamhaus.com; www.spamhaus.com/query/bl
This establishes that the sender’s IP address is on a blocklist. Some popular spam service providers are The Spamhaus Block List and Domain Block List, The SpamCop Block List, and The Passive Spam Block List.