Currently, Thunderbird does not have any capability of viewing the decrypted message source or to save a decrypted copy of a PGP or S/MIME encrypted message.
I'd like to request the addition of both capabilities. * "View source as plaintext" is needed because HTML E-Mails aren't rendered, so it's sometimes necessary to grep stuff out of the HTML source code of the encrypted message. * "Save as plaintext" is necessary to allow reporting of abuse/phishing, to add a message to other systems (DMS, CRM, ...), as well as in case of legal actions to allow providing a copy of messages as evidence (ok technically a print is often also enough, but not always) WITHOUT also having to disclose the private key every time.
From a security perspective, adding such a feature is not much different than forwarding the mail (except that forwarding loses all the headers and such). But it would greatly improve usability.
A workaround is to export the private key from Thunderbird and either share it every time a message should be saved as eml (the worst option, but currently has the best usability...) or export the key from Thunderbird and waste hours decrypting it using OpenSSL and then attaching/saving the message to something else (very very poor usability, but doesn't require sharing the private key...)
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