Unusable public key

I’ve got GPGol 0.9.92 installed in Outlook 2007 (on 64-bit Vista Business). I can decrypt mail with the plug-in, but not encrypt. Every key I try results in the error “Unusable public key”.

I have imported my keyring from PGP 8.1. I have set all my keys to Trusted in WinPT. I have even signed several (again). I still get the error “Unusable public key”.

I can encrypt via WinPT without any problems. Any ideas where I might be going wrong?