Please describe the problem:
Evolution signs the mail, attaches the signature, then encrypts the result... however, some other clients (such as Enigmail) do the encrypt & sign in a single operation.  In this case, one can export the mail and validate it using GPG, but Evolution will decrypt the message, but misses the fact that the cyphertext also included a signature.

Steps to reproduce:
Use Thunderbird+Enigmail to send a message that is both encrypted and signed.

Actual results:
The received message shows a warning that the message was encrypted but not signed.

Expected results:
The received message should correctly display the digital signature.

Does this happen every time?
Yes

Other information:
It is interesting to note that Enigmail suffers the reverse of this bug, because it goes not recursively process PGP attchments in attachments..