Review for Outgoing Message Format by Jean Lo
Rated 5 out of 5 stars
This addon solves a use case problem which some users have.
If one choose the "always HTML" add-on option, it will force Thunderbird to always send emails in that format.
Without the addon, when "always HTML" option is enabled in TB, and one sends an unformatted email, TB will send it in plaintext format.
To create your own collections, you must have a Mozilla Add-ons account.