I have an external PGP key on a Yubikey that has three identities (e-mail addresses) on it. I can send an e-mail from one of these identities to another. If I encrypt the e-mail the other mailbox will be able to decrypt it and verify the signature (of course). If I only sign the e-mail, however, the other mailbox will complain, "Invalid Digital Signature". The 'sent' folder of the sender will show the e-mail as correctly signed even when it was only signed and not encrypted. Any ideas what is going wrong here would be gratefully received.