Unable to sign message, keeps coming up

Question

An error occurred while trying to sign this message with a certificate from “***********”. Verify that your certificate for this address is correct, and that its private key is in your keychain.
<Email edited by Host>

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, or from which you're trying to send a signed message. If the message is both signed and encrypted, both addresses must match. The matching is case-sensitive: "[email protected]" does not match "[email protected]".
The signing and/or encrypting certificates 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 or sign 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.