[hashcash] MUA support through TMDA

TMDA is generally known as a challenge/response anti-spam system, but
it has other parts that can be used for more general purposes.  One
such part is a facility called tmda-inject that sits between your MUA
and MTA.  You can use this to modify the headers and envelope
of messages you send, and in this context you can use it to add an
X-Hashcash header to your outgoing messages.

This provides a MUA-independent way to tokenize your outgoing messages
which is especially useful if you use an MUA that doesn't provide a
way to do this.  This is the case with most Windows/Mac MUAs and even
many Unix ones.  TMDA supports a command-line interface for Unix MUAs
called tmda-sendmail, and an SMTP interface called tmda-ofmipd, so all
MUAs regardless of platform are covered.

For more info, see:

http://tmda.net/unstable/config-client.html
http://tmda.net/unstable/tmda-ofmipd.html
http://wiki.tmda.net/TmdaHashCashHowto

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