use cases:

fatal loss of secrets use case: Someone is sending me encrypted mail and my MUA permanently doesn't have access to the decryption secret and also not a revocation cert.

temporary second device: i don't have access to my usual MUAs, only have webmail without access/support for mail encryption. I'd like to receive and send (cleartext) mails without problems and then, after turning to use my original device/MUA again, send encrypted mails without problems.

inbome only on secondary device I use webmail most of the time. But for a small subset of my communication i want to be able to use encryption (My friend said we need this). It should not mess with my other mails though.

change to non-encrypting device: i don't use an encryption-supporting MUA anymore and want to receive/send mails without problems with peers where messages were previously encrypted, however.

setup first device/MUA:

setup another device/MUA:

loss of one out of N devices:

compose mail: i want to send a mail.

compose encrypted mail: i want to send an encrypted mail to someone.

compose unencrypted mail: i want to force my mail program to send unencrypted mail.

search in body of mails: i want to search in mail bodies (independent of their encryptedness).

validate keys: i meet one of my peers and i want to be sure there is no / has not been an attack against our encryption.

add a contact: i meet someone who also uses encryption and want to start encrypted communication with them.

deal with compromise: i suspect my encryption is compromised.