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+1. Disclaimer: This text is not an authoritative statement. If
+you are concerned about the implications of US patent 6,321,267,
+then you should give this text to your own lawyer and get their
+advice.
+
+1.1 Postfix is an MTA that aims to be an alternative to the widely
+ used Sendmail MTA. Postfix is available as open source code
+ from http://www.postfix.org/. One of the features implemented
+ by Postfix is called "sender address verification".
+
+1.2 US patent 6,321,267 (reference 4.1) describes a number of means
+ to stop junk email. One of the elements described in this
+ patent is called "active user testing".
+
+1.3 Postfix "sender address verification" and US patent 6,321,267
+ "active user testing" are implemented by connecting to an MTA
+ that is responsible for the sender address. Specifically, both
+ use the SMTP RCPT command, and both infer the validity of the
+ address from the MTA's response. Reference 4.3 defines SMTP.
+
+=====================================================================
+
+2. It is my understanding that the Postfix MTA's "sender address
+verification" does not infringe on US patent 6,321,267 for the
+following reasons:
+
+2.1 There is prior art for US patent 6,321,267 "active user testing"
+ within the context of the Sendmail MTA. See item (3.1) below.
+
+2.2 US patent 6,321,267 covers "active user testing" only in
+ combination with functions that the Postfix MTA does not
+ implement. See items (3.2) through (3.5) below.
+
+=====================================================================
+
+3. Discussion of specific details of US patent 6,321,267, and their
+relevance with respect to the Postfix MTA.
+
+3.1 Prior art. The "active user testing" method is described in
+ the paper "Selectively Rejecting SPAM Using Sendmail" by Robert
+ Harker (reference 4.2). The paper is cited as the first
+ reference in US patent 6,321,267, and was presented in October
+ 1997. The patent was filed more than two years later, in November
+ 1999. The paper says:
+
+ Bogus User Address
+
+ A desirable criterion for rejecting mail is to filter on
+ bogus user address. However, testing for a bad user address
+ is much harder because, short of sending a message to that
+ user address, there is no reliable way to check the validity
+ of the address. A simplistic test for a bad user address
+ might be to connect to the sender's SMTP server and use
+ either the SMTP VRFY or RCPT command to check the address.
+ If the server does local delivery of the message then this
+ would work well.
+
+ The prior art is about stopping junk mail with the Sendmail
+ MTA. It is my understanding that this prior art is equally
+ applicable to other MTAs, including the Postfix MTA (see items
+ 1.1 and 2.2 above).
+
+3.2 Combination of elements not implemented by the Postfix MTA.
+ Claim 1 of US patent 6,321,267 involves a combination of A)
+ determining whether the sending system is a dialup host, B)
+ determining whether the sending system is an open mail relay,
+ and C) active user testing.
+
+ Postfix does not implement elements A) and B) of claim 1.
+ Therefore, it is my understanding that the Postfix MTA does
+ not infringe on US patent 6,321,267 claim 1.
+
+3.3 Combination of elements not implemented by the Postfix MTA.
+ Claim 52 of US patent 6,321,267 involves the combination of A)
+ a proxy filter and B) active user testing.
+
+ Postfix is an MTA, not a proxy, and does not implement element
+ A) of claim 52. Therefore, it is my understanding that the
+ Postfix MTA does not infringe on US patent 6,321,267 claim 52.
+
+ US patent 6,321,267 makes a clear distinction between proxies
+ and MTAs.
+
+ Figure 13 in US patent 6,321,267 shows how a proxy interacts
+ with a sending system and a local MTA. In the case of (sending
+ system, proxy, local MTA), the proxy assumes no responsibility
+ for delivery of the email message. The responsibility remains
+ with the sending system or passes directly to the local MTA.
+
+ Figure 4 in US patent 6,321,267 shows how a sending system
+ interacts with an intermediate MTA. In the case of (sending
+ system, intermediate MTA, local MTA), the intermediate MTA
+ assumes full responsibility for delivery of the email message.
+
+ Figure 2 in US patent 6,321,267 shows how a sending system
+ interacts with a local MTA. In the case of (sending system,
+ local MTA), the local MTA assumes full responsibility for
+ delivery of the email message.
+
+3.4 The other independent claims in US patent 6,321,267 involve
+ elements that the Postfix MTA does not implement, and do not
+ involve sender address verification. Therefore, it is my
+ understanding that the Postfix MTA does not infringe on these
+ claims in US patent 6,321,267.
+
+3.5 All dependent claims in US patent 6,321,267 depend on claims
+ that involve elements that the Postfix MTA does not implement.
+ Therefore, it is my understanding that the Postfix MTA does
+ not infringe on these claims in US patent 6,321,267.
+
+4.References:
+
+4.1 Albert L. Donaldson, "Method and apparatus for filtering junk
+ email", US patent 6,321,267. Filing date: November 23, 1999.
+ http://www.uspto.gov/
+
+4.2 Robert Harker, "Selectively Rejecting SPAM Using Sendmail",
+ Proceedings of the Eleventh Systems Administration Conference
+ (LISA '97), San Diego, California, Oct. 1997, pp. 205-220.
+ http://www.usenix.org/publications/library/proceedings/lisa97/
+ full_papers/22.harker/22.pdf
+
+4.3 Jonathan B. Postel, "Simple Mail Transfer Protocol", August
+ 1982. http://www.ietf.org/rfc.html