First working version of spamprox

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+# spamprox - The barely SMTP-capable spam proxy
+
+spamprox is a really stupid proxy that just about allows
+[Postfix](http://www.postfix.org/) to use
+[SpamAssassin](https://spamassassin.apache.org/) as a
+[before-queue content filter](http://www.postfix.org/SMTPD_PROXY_README.html).
+
+It speaks enough SMTP
+* to send a "220" greeting
+* to respond to everything up to a "DATA" command with "250 Ok"
+* to respond to a "DATA" command with "354" and then wait for the terminating "."
+* to pass the resulting email to SpamAssassin and respond with a "250", "554" or "TODO" result
+* to respond to a "QUIT" with a "221 Bye"
+* to respond to anything else between "DATA" and "QUIT" with "503 Bad sequence of commands"
+
+Fortunately Postfix is already doing general validation on the incoming mail
+from the internet, so we should be able to trust that the input follows a
+consistent SMTP command order that we can predict, and not have to worry too
+much about being subverted from that direction.
+
+Still, this should probably be written in `python3`, or some other language with
+a proper SMTP library. But this was my 4th attempted approach at getting Postfix
+to be able to reject (rather than bounce) spammy emails and I was in the mood
+for an ugly hack. Also, when I started I thought I'd just be able to slurp in
+the whole command stream and pass a single "250"/"554" back at the end. I
+figured the whole thing would be about 10-15 lines long, which is OK for a
+shell script.
+
+I didn't realise that Postfix wanted a proper SMTP conversation with the proxy,
+and by the time I did realise it I was the first 90% of the way there. I
+probably *should have* realised it from the start, but hey, learn something new
+every day. Heck, when I realised that I needed to clear IFS (the
+[Input Field Separators](https://manpages.debian.org/stable/dash/dash.1.en.html#ENVIRONMENT)
+special variable) to get the thing to work, that should have been a big flag to
+turn around and rewrite it in another language, but at that point I was the
+*second* 90% of the way there and I didn't want to throw away the work...
+
+Oh yeah - writing it in a langauge that allows it to run as a proper daemon
+might be a good idea too. Instead of it just being a script that runs off of a
+`systemd`
+[socket unit](https://manpages.debian.org/stable/systemd/systemd.socket.5.en.html),
+or an `(x)inetd`
+[service](https://manpages.debian.org/stable/xinetd/xinetd.conf.5.en.html).
+
+Either that, or implement the
+[spampd](https://manpages.debian.org/stable/spampd/spampd.8.en.html#To_Do) TODO
+of "Add configurable option for rejecting mail outright based on spam
+score." so I can just use that (as I at one time had planned to) instead.
+