Signing emails only works intermittently - problem reproducible

Question

Good Afternoon,
I am two digital certificates from VeriSign/Symantec for use with my email. One is for digitally signing email, the other for encrypting email. I have both certificates installed in my login keychain and they are showing up in the "My Certificates" section.  I also have root and intermediate certificates for VeriSign installed in my login keychain.
When I open the Mail application and attempt to send a digitally signed email sometimes I have the option and sometimes it is grayed out. The problem is reproducible in an unusual way. If I am looking at my inbox and select an email in my inbox, any email, and then click "Compose" I am UNABLE to digitally sign the new message. If I am looking at my inbox and I don't have any message selected and click compose, I AM able to sign the email. Please note, I am not replying to a message, only creating an entirely new message.
I have been able to successfully send signed messages using the second method listed above, so I know my certificates work. I have removed the certificates, repaired the keychain, and reinstalled the certificates, and repaired the keychain again. No problems are ever reported with the keychain. I am restarted the mail application numerous times and restarted the system several times. I updated the OS and all software to the latest versions available. Here are some details about my system:
Macbook Pro
Mail application version 5.3 (1283)
OS X 10.7.5
Also, two other individuals are having the same problem in my organization when using Macs, so I don't think this issue is due to my specific hardware configuration or my certificates. Thank you for your help.
-Trevailious

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.
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. Test.
If the test fails, delete all the certificates again, then reinstall them from fresh copies.