eM Client doesn’t allow encrypted e-mails to be searched, even though a certificate is installed and I can read the content just fine. I realize that this is common behavior among mail clients, and the need to search encrypted mails is what brought me to eM Client in the first place - if you were to implement this, I’d leave whichever client I’m using for eM Client in a heartbeat.
My reason for using encryption is for transit; I want them to be secure from sender-to-recipient. Once they’re on my hard drive, I can be responsible for my own security, and I really believe these two behaviors should be distinct. Perhaps the solution is an option to i.e. “decrypt encrypted e-mails on arrival, and store as unencrypted,” thus allowing them to be searched like any other.
(Virtually every client I’ve tried already has requests for this feature - including several crowdfunded offers for Thunderbird, but bafflingly, nobody offers it. Please be the first! :))