I had a similar issue. It ended up being an add-on[1]. Once I disabled the add-on, I could send with no problem.
[1] https://addons.thunderbird.net/en-us/thunderbird/addon/check-and-send/
I had a similar issue. It ended up being an add-on[1]. Once I disabled the add-on, I could send with no problem.
[1] https://addons.thunderbird.net/en-us/thunderbird/addon/check-and-send/