Comment 88 for bug 52667

Thunderbird doesn't support RFC-2369 based Reply-To-List and because of that people use Reply-To-All is a substitute. This causes people to get double copies of the same mail which can be annoying (resulting in mailing list flamewars etc. ;-) ).

The Thunderbird 3.0 branch has a fix for this, but that's not very useful (it will be at least a year before it gets released). Recently someone who wrote an extension that uses the fix also made a patch for the 1.5 and 2.0 branches though, so I think we should include this into edgy (and maybe later to dapper backports/updates?). Provided the patch doesn't break other things of course...

http://open.nit.ca/wiki/index.php?page=ReplyToListThunderbirdExtension