Linux – Email client for Linux which can store GPG messages decrypted

try something like

gpg -ea -r "Recipient name" -o - filename | mail -s "Subject line" recipient@example.com

to send an ascii-armored, public-key-encrypted copy of the file "filename" to a person named "Recipient name" (who is in your gpg keyring) at email address recipient@example.com with the specified subject line.

or

echo "Your secret message" | gpg -ea -r "Recipient name" | mail -s "Subject" recipient@example.com

to send text directly rather than from a cleartext file on disk.

Have you tried BCC'ing yourself and adding a per recipient rule that says to never, sign or encrypt the message to your BBC'd email address?

Then on the incoming you could make a rule, but you would have to provide your passphrase interactively anyway, so you might as well copy the email to a folder after you decrypt it.

Of course, if you had access to the mail server, and you run maildir, you could always write a short script that greps the mail files looking for "-----PGP BEGIN -----" and runs them through GPG on the command line and them copies them to a folder. Then you could just set it up for a cron job. Caveat is that you would have to provide the script your key and passphrase - which sort of defeats the whole idea, but possible.

/2 cents

UPDATE: I was thinking (this morning in the shower...) that since you're running linux as your OS, you could just write a shell script that greps your thunderbird inbox (and folders) to find the PGP files on the command line, decrypt them, and then copy them in to the appropriate mail folder. That way none of the data is in the clear on the server. You could run the script interactively for better security.