Sendmail\Postfix: Sign every email that goes through with X.509?

Question

postfix

There is an external application that generates letters and sends them via sendmail\postfix. In this case, a Rails application.
Is it possible to automatically sign certain emails with an X.509 certificate using sendmail\postfix?

Answer 1

Both sendmail and postfix support the milter protocol for mail filtering ( www.milter.org ). A similar approach is used, for example, by the DKIM signing filter (http://en.wikipedia.org/wiki/DKIM), though not x509.
So you can write your own filter to solve this problem. There are a number of libraries for working with the protocol. For example, in python ( pymilter ), then in your filter you can use the interface to OpenSSL ( pyOpenSSL ) with which to sign some fields.
If python is not suitable, then you can write a filter in perl ( Sendmail::Milter ) or in C ( milter.org API )