How do I configure Thunderbird to send plain text emails by default?

Solution 1:

As @enzotib pointed out in the comment, this is set up in the Account Options, under "Composition and Addressing"

Then Thunderbird will compose emails in plain text. To switch from plain text for one email to HTML, hold shift while you click on Reply, Forward, or compose and it will let you compose in HTML.

Solution 2:

To stop Mozilla Thunderbird from asking you about the desired format when you compose a rich text message and click Send:

1. Select Tools - Options (or Thunderbird - Preferences ... on a Mac) from the menu.
2. Go to the Composition category.
3. Make sure the General tab is selected.
4. Click Send Options...
5. Under Text Format, make sure something like Ask me what to do is NOT selected.

I recommend Send the message in both plain text and HTML, which preserves any rich formatting while giving recipients a chance to choose a plain text alternative.