I tried out this addon, and it is an awesome security improvement given Thunderbird for some reason entirely ignores the Authentication-Results header!

However, when there's no DKIM header and an SPF softfail remarked in the Authentication-Results, then this addon currently just displays nothing. Instead of that, it would be nice if it actually displayed the SPF softfail since that seems like a pretty significant warning.

The addon options could maybe look something like this to make that easily configurable:

[ ] Verify DKIM signatures locally
[ ] Save result of the local verification
[ ] Show server DKIM check (Authentication-Results header)
[ ] Show server SPF check (Authentication-Results header)

I don't really care what the defaults are, but it would be nice if SPF results could be enabled this way.