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Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+[The patent discussed in this document expired in 2019, without a
+request for extension. The owner of that patent can no longer sue for
+infringement. However, other patents may make similar claims. The text
+below may serve as an example for dealing with them.]
+
+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