[imp] unchecked signature in encrypted message because public key not found

[Maciej Uhlig]

I'm testing PGP functionality of IMP 5.0.17 with HGWE 4.0.5. If signed
and encrypted PGP/MIME message comes from another contact than Horde/IMP
user, I get error message from gpg:

gpg: Can't check signature: public key not found

It's important to say the message is correctly decrypted, so Horde user
public/private key pair with passphrase is in place.

Looks like _sender_ public key is unknown for gpg. However the key is
present in the contact addressbook entry and at public keyserver. It is
also present in Horde PGP Public Keyring.

If only signed message is sent from this contact to Horde user, message
from gpg is correct:

gpg: Good signature from ...

So, I'm not sure what is (should be) the source of the contact public
key: is it an addressbook entry , PGP Public Keyring, or is it a result
of gpg keyserver query? What could be a problem (bug? configuration
issue?) here? Why sender public key is not found in case message is
encrypted?

Best regards,

MU