[Enigmail] Expect signature header proposal

[Eitan Adler]

[snip]
Alright - what about this scenario:

Bob is security conscience and always signs his messages sent to Alice. 
  Dora wants to harm Bob's career and she sends a forged message 
claiming to be from Bob to Alice.  Alice is an ordinary human and does 
not notice the missing "this message is signed" notice and therefore 
acts upon this message.  Bob's career is ruined.  Alice also gets 
messages from many people that don't sign their emails.  It would ruin 
the effect of an "unsigned message" notice if she got it for every 
message that had no signature.

Even if it is not a header there should be some way to enable/disable a 
"unsigned message" notice per user.