Mavericks: cannot decrypt S/MIME email

Question

Mavericks 10.9.4 on MacBook Air, using latest Apple Mail v7.3 (1878.6).
I can send and receive (verify) signed S/MIME email, and send encrypted S/MIME email.
Problem: I cannot read received encrypted email, even email that I myself send to my own address (and for which I obviously have all the appropriate keys). Encrypted email arrives as if it wasn't secured at all, and its only content is attached file "smime.p7m".
I have my own CA, which issued certificates for a few of my email accounts. However this problem manifests itself also on accounts whose keys are certified by my school.
This email and these certificates worked with Mountain Lion...
I'd appreciate any help, and would be happy to provide any info that might help finding a solution to this problem.
Thanks!
Here's an excerpt from the headers from one of the encrypted messages:
Content-Type: application/pkcs7-mime; name=smime.p7m; smime-type=enveloped-data
Mime-Version: 1.0 (Mac OS X Mail 7.3 \(1878.6\))
Content-Disposition: attachment; filename=smime.p7m
In-Reply-To: <[email protected]>
Content-Transfer-Encoding: base64
Message-Id: <[email protected]>
References: <[email protected]>
X-Mailer: Apple Mail (2.1878.6)
X-Score-Level: *** 3.124
Re: Test (signed encrypted)
Message was edited by: Mouse008
Added message headers.

Answer 1

First, the address associated with the S/MIME public key must exactly match the address to which you're trying to send the encrypted message. The matching is case-sensitive: "[email protected]" does not match "[email protected]".
The recipient's certificate must be valid: not self-signed, expired, or revoked. You can check the status of the certificate in Keychain Access (see below.)
If you can't encrypt messages to a valid address with a valid certificate, continue.
Back up all data before proceeding.
Launch the Keychain Access application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Keychain Access in the icon grid.
Select the login keychain from the list on the left side of the Keychain Access window. If your default keychain has a different name, select that.
If the lock icon in the top left corner of the window shows that the keychain is locked, click to unlock it. You'll be prompted for the keychain password, which is the same as your login password, unless you've changed it.
Right-click or control-click the login entry in the list. From the menu that pops up, select
          Change Settings for Keychain "login"
In the sheet that opens, uncheck both boxes, if not already unchecked.
From the menu bar, select
          Keychain Access ▹ Preferences... ▹ First Aid
If the box labeled Keep login keychain unlocked is not checked, check it.
Select Keychain from the menu bar and repair the keychain.
Quit and relaunch Mail. Test. If the problem isn't resolved, continue.
Export all S/MIME certificates, delete them from the keychain, and reimport. For instructions, select
          Help ▹ Keychain Access Help
from the menu bar and search for the term "export" in the help window. Export each certificate as an individual file; don't combine them into one big file.
Test again. If the test fails, delete all the certificates again, then reinstall them from fresh copies.