User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; Q312461; (R1 1.1); .NET CLR 1.1.4322) Build Identifier: version 0.6 (20040502) If you compose HTML-Mails, you can switch to plaintext, but it is not directly converted. You'll have to send the mail and it is converted automatically. You have no chance to recheck how it looks like. If you configure Thunderbird not to use HTML for mails, you cannot switch to plaintext, so this is not even a better choice. The behavior of eg. Outlook is much better. :-( Reproducible: Always Steps to Reproduce: 1a. configure Thunderbird to enable HTML-Mails 1b. try to write a plaintext-only mail and check how it looks like before you send it 2a. configure Thunderbird not to use HTML 2b. try to change the message format to HTML when you write your mails Actual Results: You'll see that this is not very useful. Either you use HTML and you cannot control how your plaintext mails look like before you send them or you cannot use HTML for mails if you preconfigure plaintext. Expected Results: It should behave like Outlook. If I switch from HTML to plaintext and back, it reformats my mail instantly. Or you can switch between plaintext/ HTML-view and reformat each before you send it. (eg. if you send both formats together in one mail) This should be tested in combination with the enigmail-plugin.

This bug is *not* trivial. *** This bug has been marked as a duplicate of 216132 ***