User Agent: Mozilla/5.0 (X11; Linux i686; rv:19.0) Gecko/20100101 Firefox/19.0 Build ID: 20130307023931 Steps to reproduce: 1. I sent an email to a friend (using another email software, namely `mutt`). - this mail contained a text/plain attachment - the attachment started with the word "From" (actually, it was: From here to there. but without leading spaces (see attached file)). 2. My friend received the email with Thunderbird. 3. He saved the attachment to disk. 4. He opened the saved attachment with a regular text editor. Actual results: In the text editor, the attachment was shown starting with ">From". (Prepended with ">"). Expected results: It should be shown starting with "From". (Just as it was sent, without ">"). HINT: It seems to me that this bug may result from using a mail receiving library which automatically prepends all "From " lines in a message body with ">". Such practice might have been introduced (in the library) for the benefit of traditional Unix mbox format. Cf. `man mbox`: Example: >From example@example.com Fri Jun 23 02:56:55 2000 In order to avoid misinterpretation of lines in message bodies which begin with the four characters "From", followed by a space character, the mail delivery agent must quote any occurrence of "From " at the start of a body line.

So this seems as correct behaviour according to the "standard". However this is already filed as bug 121947 to find a workaround.