Description of Problem:

When a message is encrypted and signed with GnuPG, Evolution asks your
passord to decrypt it, but it also seems to check the validity of the
signature. So, if you haven't got the sender's public key in your keyring,
you won't be able to read the message. An error appears (something like
"sender identity could not be checked" - gnupg warning).
Also, if you have the public key, there's no lock icon at the bottom of the
e-mail, the signature is automatically checked and you won't be notificated.