Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2.14) Gecko/20110223 Thunderbird/3.1.8 on Ubuntu Lucid I have user_pref("mailnews.plaintext_domains", "docomo.ne.jp,debian.org"); in my prefs.js. Yet, whenever I write a mail to @debian.org or @bugs.debian.org it is sent as HTML mail.

Isn't is send as both (ie html + text ) ? Does this works better in -safe-mode (see http://support.mozillamessaging.com/en-US/kb/Safe+Mode) ?

already exists as bug 401014

no, not the same bug.

need more info. this feature does work in general (see bug 401014, where it actually matches too often).

my apologies for the delay in responding. The mails are sent out *only* as HTML. One example is http://bugs.debian.org/cgi-bin/bugreport.cgi?msg=10;att=0;bug=624592 for which http://bugs.debian.org/624592 has the full mbox folder, so you can have a look at the details. Let me know if there is any other information that I can provide.

Rolf what extensions are you using (ie BiDi) ?

I normally have the following active extensions for TB: - Archive This - Compact Header - Enigmail - Mail Tweak - Message Archive Options - Unified Search - Virtual Identity And indeed, it seems that one of those extensions is the culprit rather than TB itself. If I start TB with all extensions off, I believe TB behaves as expected. I shall reconfirm.

the Mail Tweak plugin is the "culprit", my apologies for the noise. I'll contact the maintainer of the plugin.