06321267 is referenced by 455 patents and cites 14 patents.

An Active Filtering proxy filters electronic junk mail (also known as spam, bulk mail, or advertising) received at a Message Transfer Agent from remote Internet hosts using the Simple Mail Transfer Protocol (SMTP). The proxy actively probes remote hosts that attempt to send mail to the protected mail server in order to identify dialup PCs, open relays, and forged email. The system provides multiple layers of defense including: connect-time filtering based on IP address, identification of dialup PCs attempting to send mail, testing for permissive (open) relays, testing for validity of the sender's address, and message header filtering. A sender's message must successfully pass through all relevant layers, or it is rejected and logged. Subsequent filters feed IP addresses back to the IP filtering mechanism, so subsequent mail from the same host can be easily blocked.

Title
Method and apparatus for filtering junk email
Application Number
9/447590
Publication Number
6321267 (B1)
Application Date
November 23, 1999
Publication Date
November 20, 2001
Inventor
Albert L Donaldson
Oakton
VA, US
Agent
Blank Rome Comisky & McCauley
US
Assignee
ESCOM Corporation
VA, US
IPC
G06F 15/16
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