[exim] Digitaly sign emails through a transport_filter

Hello exim specialists.

Today i got an idea - why not sign every outgoing email identified by smtp
auth.

My thoughts are:

1. Send email through some email client (or webmail, whatever);
2. Exim checks authentication;
3. If authenticated correctly, proceed with 4. otherwise go to 5.;
4. Exim change the email and sign it, maybe through an external filter app
using transport_filter;
5. send it out to the recipient address;
6. recipient got an 100% valid email from me;

That way i dont need to care about signing my email using some 3rd party
tools in outlook or other email programs and always have my outgoing email
signed.


On the way looking for such a solution i found different ideas:

1. Use GnuPGP = not included within most email clients, PGP is required to
verify;
2. Use s/mime (found x-pkcs7-signature in header);

The second idea seems to work as in outlook for example the email comes with
a "signed" mark.
But i may need some signing provider like verisign, not?


I am not clear about the way how to start so if someone could give me a
litle push...

Any ideas on this?



Best wishes,
Juergen