Windows 7 Pro, Office 2010 Pro.  I have three IMAP accounts on my laptop.  I have three digital certificates from Comodo, one for each account.  I have been using these certificates with these accounts since April, signing all messages and encrypting messages from one account.  All three certificates are valid until 2020.

Two days ago I started getting an Invalid Certificate message (Microsoft Outlook cannot sign or encrypt this message because there are no certificates which can be used...) for every email I sent from each of the three accounts.

When I click on Change Security Settings button I see that Add digital signature is checked (as it should be).  Security Settings are set to <Automatic>.  When I click on Automatic I can see all three mail accounts listed in the form My S/Mime Settings (email address).  If I attempt to change the security setting from Automatic to one of the account- specific S/Mime Settings the Invalid Certificate message immediately pops up again.

If I click the Change Settings button and then click on the button to choose a Signing Certificate then I get an error message statiing No Certificates Available.

At File/Options/TrustCenter/Trust Center Settings/Email Security when I click on Import/Export Digital Certificates and attempt to Export your Digital ID to a file by clickiong on Select... I again get the No Certificate Available error message.

Any idea how to get Outlook back to working with my certificates?

BTW: I set up my email accounts with certificates and encryption because my wife and I were having a major problem cyber attacks on our PCs and with MITM attacks on our email.  I am concerned that my PC may be suffering a relapse...

Check the following: Check Tools | Trust Center | E-mail Security and make sure that "Encrypt Contents and attachments" isn't checked. Also better to have a look at the article: http://support.microsoft.com/kb/974334

Thanks, was successful.

This is not even an answer. The poster didn't mark this as an answer but the Microshaft staff moderator did. They probably didn't even read the question. Unchecking the Encrypt Contents and Attachments defeats the purpose of having the certificate in the first place. This is obviously Microshaft just trying to mask the symptom and not providing a real answer then marking it as an answer themselves. I bet this guy Chen marks thousands of answers a day but doesn't even read them. Microsoft security... What a joke.