Version:           4.7 (using KDE 4.7.3) 
OS:                Linux

Here is an example of incorrect report:

Encrypted message
Message was signed on 19.12.11 21:29 with unknown key 0x18FCD1311DCF84834BED9F857027404925B7C18D.
The validity of the signature cannot be verified.
Status: Good signature	Hide Details
Test: Signed, Encrypted
End of signed message
End of encrypted message


In fact 0x25B7C18D key is a signing subkey of my main key 0x46EB581F. As far as I understood kmail does not identify mail only if combined method described in p 6.2 of RFC 3156 is used for encrypted&signed mails. For example, combined method is used by Enigmail Thunderbird extension.

If non-conbined method is used then all will be displayed correctly, in this case kmail identifies main key by subkey.

Reproducible: Always

Steps to Reproduce:
1. Send encrypted&singed PGP/MIME email message e.g. using Thunderbird + Enigmail to yourself. Use a key that has a signing subkey and ensure that subkey used for signing.
2. Try to decrypt and verify the signature.

Actual Results:  
Encrypted message
Message was signed on 19.12.11 21:29 with unknown key 0x18FCD1311DCF84834BED9F857027404925B7C18D.
The validity of the signature cannot be verified.
Status: Good signature	Hide Details
Test: Signed, Encrypted
End of signed message
End of encrypted message

Expected Results:  
Encrypted message
Message was signed by mail@++++++++++++.com (Key ID: 0xA0F213F146EB581F).
The signature is valid and the key is fully trusted.	Hide Details
Test: Signed, Encrypted
End of signed message
End of encrypted message