Excellent integration in Thunderbird Rated 5 out of 5 stars
Contrary to what some commentators might think, and especially hanzolo, this extension works perfectly (except perhaps the bug reported by alex dean but I never faced it). It shows itself in the UI, but unlike too many extensions, it doesn't show a big new button, but rather integrates very elegantly with the existing Send Message Now button by adding a small drop-down arrow on it, allowing to choose the SMTP in just one more clic. No options needed, it just works out-of-the-box!
I really appreciate not needing to first click on a separate button to choose the SMTP. If you want a big button in your UI, see the "SmtpSelect" extension.