If we read protected headers and replace the subject we also need to encrypt the subject when replying / forwarding the mail :-/ otherwise we would "leak" the subject.

We also need protected headers to properly handle the BCC split case T4814, and if we create a protected-headers mime part anyway it's not a large step to have an option to encrypt the subject, even though we think that this breaks E-Mail :-/

Rev. 080f5bd701f13b1a714111eed206c2d9a4cdfb30 and e6fbea5b0049667e6bc4be3ca13e4f767ed3acbc implemented most of this already.