Digital Signatures and Encryption in Yosemite Mail

Question

After upgrading to Yosemite, I am having difficulty using the Mac Mail app to send digitally signed and encrypted email.
Before the upgrade to Yosemite, I was able to send signed and encrypted emails using certificate/keys in my keychain using both the Mac Mail app and Microsoft Outlook 2011 for Mac.
After upgrading, I am still able to send signed and encrypted message in Outlook, but the Mac Mail app gives the following error when I attempt to send a signed email:
'You don’t have a trusted certificate in your keychain that matches the email address “XXXX@XXXX”. Without a certificate, you can’t sign messages sent from this address.'   (Actual name replaced)
When I look at my certificates in my keychain, a certificate is available with "Usage: Digital Signature" that has the email address from the error message "XXXX@XXXX" with exact case in the RFC 822 Name.
Another interesting piece of data that might help track this down is that when I first launch the Mac Mail application, the Mac Mail application is able to successfully decrypt emails that have been previously sent encrypted to me.  HOWEVER, after I attempt to send an email and get the "You don’t have a trusted certificate..." error message, these emails are no longer able to be decrypted.  I get the "Unable to decrypt message" header above the message and the content of the message is just a "smime.p7m".  If I close the mail application and restart it, these encrypted message are once again decrypt-able until I attempt to send a message.
It almost seems like things are working until mail tries to access the keychain.
I have attempted to delete my certificate and keys from my keychain and then adding those items again.
I have attempted to close the mail application and reopen it.
I have attempted to reboot my computer.

Answer 1

1.  I want to confirm that this is still an issue for me in 10.10.1 and mail Version 8.1 (1993)
2.  I have another data point.
At my office I have wired networking and wireless networking available.  Primarily I utilize the wired networking for access to network drives, etc.
When using the wired networking, I experience all the problems that have been catalogued in this thread.  Can't sign, can't encrypt, can't close the compose window after the mail program fails to find my certificate.
However, when I switch to wireless networking before starting the mail application, digital signatures and encryption seem to work!  This is pretty weird behavior.  Make sure to restart mail if you were previously wired.
Here are some theories:
Something to do with OCSP?  When I am wired vs wireless I am on different ip subnets and subject to different firewall rule sets.  Perhaps OCSP is trying to determine the status of the certificate and failing? 
Here are some things I have tested:
I switched to a different official apple brand thunderbolt to ethernet adapter with no change in behavior
I disabled wireless and disconnected my wired network.  So no network access at all.  Signatures and encryption work!  The message obviously does not send, but it appears in my outbox and I don't get the signature error.  When I reconnect my wired cable, the message sends successfully and appears as encrypted in my sent folder!
I have attempted to disable OCSP by using "Keychain Access --> Preferences --> Certificates Tab --> OCSP (OFF) and CRL (OFF)" but this hasn't made a difference in the behavior of wired networking.
Ran a TCPDUMP on traffic to the OCSP service but didn't see any traffic when I attempted to send a message and received the signature error
I am pretty stumped on this.  This is very odd behavior
Does anyone else experience this behavior?