[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
