Hey there, Mike.
Did you ever secure a good answer to this inquiry? If not, I believe that the underlying certificate management framework handles that for you. If I understand things correctly, the Certificate Authority issued the Digital ID that is associated
to a specific email account. I too run multiple email accounts in my outlook client, and if I try to sign an email that is being sent from an account that is different than the one that the Digital ID is for, the client tosses an error dialog indicating
that the digital id is not valid for said account. As such, I think that your concern is mitigated by default.
The hurdle is configuring the client to try to use the certificate only when sending emails from the account associated with the Digital ID. I ended up, in Trust Center|Email Security defaulting to signing each email and then I customized the ribbon
to put the sign/encrypt buttons on the default MESSAGE tab, as reflected here: http://i.imgur.com/z37sj5j.png (note the permission section on the far right).
That way, if I am going to send from an account that is not associated with the Digital ID, then all I have to do is to stroke the Sign button once to tell the client not to attempt to sign said email being composed.
Hope that helps, if you're still looking for a resolution.
...
What brought me to this thread is my desire to know if Outlook, and the underlying OS, supports multiple Digital IDs. As I drafted this, I talked myself into acknowledging that it probably does by being able to import multiple certificates and then
relying on the integration between the client and host OS to handle selecting which ID/Certificate is appropriate for use when a specific email is composed and sent (based on the originating account) with Signing (& Encryption) enabled. I am going
to get another Digital ID and put the theory to test. I suspect that it'll be intuitive and work just like one would expect. Wish me luck.
Take care.
-t