Seth Ladd had an interesting idea: adding an x-foaf header to outgoing emails. I wondered if this was possible in Thunderbird and after a bit of digging I found that you can do it by adding the following to prefs.js in your Thunderbird profile directory:

user_pref("mail.identity.id2.headers", "foafuri");
user_pref("mail.identity.id2.header.foaf", "X-FOAF: http://purl.org/NET/iand/foaf");

Each identity in Thunderbird has its own set of preferences in that file, so you need to add those two lines for each one. Change the id2 to the appropriate id number for that identity.

I originally chose to use X-FOAF-Uri rather than Seth's suggestion of X-FOAF since it might be useful to add other FOAF related mail headers. However, there was some discussion in the SWIG IRC channel yesterday and it looks like X-FOAF is more widely used, so I'm changing the header back to that. An IFP like X-FOAF-Mbox-Sha1sum would be a good idea to assist locating the right person in the FOAF file.

Updates 10 December 2004:

Suzan Foster worked out how to do the same for Mail.app: quit Mail.app, open up Terminal and type something like:

defaults write com.apple.mail UserHeaders '{"X-FOAF" = "http://www.xs4all.nl/~foz/foaf.rdf";}'

Read Suzan's full post for some caveats on overwriting existing headers.

In the comments, Darren Chamberlain suggests this for Mutt:

send-hook . 'my_hdr X-FOAF: http://purl.org/NET/iand/foaf