[Macpartners] When I open an email in mac mail, it opens a window which says "Mail wants to sign using key privatekey in your keychain".

[Mark London]

I have a user using mac mail, and they received an email from someone on 
campus (which was sent from owa.mit.edu).  When that email is opened, 
another window pops up, which says "Mail wants to sign using key 
privateKey in your keychain" "Do you want to allow access to this 
item".  "Keychain: login.keychain, Application: Mail", and the button 
options are Always Allow, Deny, or Allow.  Can anyone tell me why mac 
mail is doing this, for this specific email?  It doesn't happen on other 
emails that the user reads.  Thanks. - Mark