Can you try

doveadm -o plugin/mail_crypt_private_password=desired_password mailbox > cryptokey generate -u user -UR

Aki

I tried that and got the following:

user@host:~$ doveadm -o plugin/mail_crypt_private_password=desired_password mailbox > cryptokey generate -u user -UR

Folder Public ID

user@host:~$

Then I sent a new email to the mail server, and I checked it with mutt but got 'Internal Error' 'error opening mailbox' and I checked the email with Roundcube and I can see the subject header but when I open the email its just blank.

Sent with ProtonMail Secure Email.