Filter privacy-sensitive headers on outgoing mail

This re-implements part of PR #69 by @mkropat, who wrote:

By default, Postfix adds a Received header — on all mail that you send —
that lists the IP of the device you sent the mail from.  This feature is
great if you're a mail provider and you need to debug why one user is
having sending issues.  This feature is not so great if you run your own
mail server and you don't want every recipient of every email you send
to know the device and IP you sent the email from.

To limit this filtering to outgoing mail only, we apply the filters just
to the submission port.  See these guides [1] [2] for more context.

  [1] http://askubuntu.com/a/78168/11259
  [2] http://www.void.gr/kargig/blog/2013/11/24/anonymize-headers-in-postfix/

conf/postfix_outgoing_mail_header_filters

@@ -0,0 +1,10 @@
+# Remove the first line of the Received: header. Note that we cannot fully remove the Received: header
+# because OpenDKIM requires that a header be present when signing outbound mail. The first line is
+# where the user's home IP address would be.
+/^\s*Received:[^\n]*(.*)/         REPLACE Received: from authenticated-user (unknown [127.0.0.1])$1
+
+# Remove other typically private information.
+/^\s*User-Agent:/        IGNORE
+/^\s*X-Enigmail:/        IGNORE
+/^\s*X-Mailer:/          IGNORE
+/^\s*X-Originating-IP:/  IGNORE