Note
To be sure the "passphrase agent" (gpg-agent) is enabled, check your ~/.gnupg/gpg.conf and eventually uncomment the line containing "use-agent".
In KMail's , select , then . On the GPG Agent page, give a path to which logs can be written (), in case of problems. You can check the log in KMail's . It's probably a good idea, too, to increase the cache time - I use 3600.