I got encrypted mail, yay! I noticed that the sender had helpfully used autocrypt, because there was a header:
Autocrypt: email@example.com; keydata= ABC... DEF...
How to extract this? I don’t know. I know that with Delta Chat, this would have just worked. I noticed that Emacs has a package implementing autocrypt, but I don’t actually want to setup an account and I don’t understand all the extras it does (gossip‽) – so I wanted to do it manually.
I saved the keydata in a file and decoded it, ignoring the whitespace at the beginning of each line:
base64 --ignore-garbage --decode test.txt > test.key
And imported it:
gpg --import test.key