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I would like to gather some general information and your experience how
email clients behave by default when you press REPLY button for an
encrypted mail. Just a general picture and your experience.
One datapoint you might find interesting... I worked at a US defense contractor that was a Windows shop, but used Thunderbird. Thunderbird was used because it could be configured to encrypt and sign all emails by default. And that's how the company was configured -- we had to do something special to send a non-encrypted email.
The biggest pain point came when searching emails. As you probably know, most encryption schemes break sorting and searching. Sorting and searching can be accomplished on encrypted messages, but the system needs to use Homomorphic Encryption (HE) or Semi-Homomorphic Encryption (SHE) schemes. Those schemes are still in the lab and considered experimental.
Jeff