[Enigmail] Expect signature header proposal

[Eitan Adler]

I think the following are fairly well accepted assumptions:
1) People generally notice the absence of a notice
2) Not everyone uses encryption
3) Even among those who do not everyone uses it all the time.

When someone that does use encryption on a regular basis doesn't sign a
message most people do not notice.  I propose some kind of extra mail
header such as "X-Expect-Sig-Always" be set if you plan on always
signing your messages.
Then if you receive a unsigned message from someone who has previously
set that header Thunderbird show an obvious warning such as "This
message is unsigned.  Sender has notified you that all messages should
be signed." or something to that extent
--
Eitan