Probably a bug: double-clicking on a stored eml file tlls, it cant'decrypt it - even after TB has got the password
This is probably a bug in Thunderbird 91.5.0 (64-Bit) on Xubuntu 20.04:
Wehen I double-click on an mail, which I have stored with TB, TB opens and asks me for my password.
Then it opens the stored email (one, which I had sent recently, a non-encrypted signed one). But TB does not show the content of it but rather a box with:
Dies ist eine verschlüsselte OpenPGP-Nachricht, aber Unterstützung für OpenPGP-Entschlüsselung ist nicht verfügbar. (decryption is not available)
However, if I uses the same session of TB to open the *.eml-file in Question by the File>Open file dialogue, I can open it.
BTW: Looking into this file with Geany, I see among other parts (including the plain text)
1. a block Content-Type: text/plain; charset=UTF-8; format=flowed, Content-Transfer-Encoding: base64 followed by base64-data (character garbage), 2. a PGP PUBLIC KEY BLOCK with by other character garbage, 3. my PGP Signature block with by other character garbage.
Apparently all these parts are used meaningfully when I open the particular file through the File>Open file dialogue, except that the mail is not indicated as a signed one, which it most probably was since I always sign outgoing emails. That's probably another bug.
When I double-click again on the same file in Thunar, while it already has been opened by TB showing the above mentioned error message, it opens correctly like above.
However, if I open TB in the normal way, i.e. not by double-clicking on a stored email, I am first asked for my password (as expected). If I open the stored file then by File>Open file it shows the email as if I had opened it from the "sent" storage, i.e. with the symbol that it is a signed email and tools to show more about the applied key.
I suppose this is a glicht which can be easily corrected by internally following the same sequence of events for the file after a double click on it in the Thunar file browser and then perform the same steps after the password dialogue ais if the file had been selected by a file open dialogue.
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Perhaps try on the encryption mailing list. https://thunderbird.topicbox.com/groups/e2ee