[MlMt] Warn sending encrypted mail about unencrypted subject

[Bram Heerink]

>> Would it be nice to warn users that are trying to send encrypted 
>> e-mail about this with a "don't warn me anymore" option?
>
> I'm not sure how many encryption users are unaware of this limitation 
> (especially MailMate users). I'll note the idea, but do any other 
> email clients warn about this?

I think (most) people do know when they think about it. But are you 
aware enough all the time? Reason I suggested this change is I composed 
an encrypted e-mail and then remembered 'oh subject is not encrypted' 
and then I changed it. But true, it's not a big thing, I think.

Don't know any mailclient warning about unencrypted subject.

~bram