No encryption of all messages stored on vendors' servers (e.g. Outlook, Google and others)
Will and possibly Thunderbird allow encryption of all messages stored on vendor servers (e.g., Outlook, Google and others), such as archived messages and all others that were not encrypted sender-receiver? Will and possibly when will integrated PGP enable encryption of all messages, including those not encrypted during communication, with a private PGP key?
Opaite Mbohovái (1)
Will and possibly Thunderbird allow encryption of all messages stored on vendor servers (e.g., Outlook, Google and others)
Messages stored on the email provider's server (data at rest) can only be encrypted by the party controlling the server, i.e. by the email provider. Thunderbird does not control the email provider's server.
Will and possibly when will integrated PGP enable encryption of all messages, including those not encrypted during communication ...
You'd have send messages encrypted in the first place. Also, you can tell Thunderbird to store drafts encrypted. If possible at all, I'd assume TB would only be able to encrypt messages stored locally. Whether something like this is planned at all I don't know.
... with a private PGP key?
The public key is used for encryption.