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