[exim] [PATCH] add verify=header_sender[=header_type] functi…

Nothing like a new release to remind me about the pain of migrating
custom patches to each release. If you would like to incorporate this
in a future release, please do.

I wanted specify which header sender field to verify, I've expanded the
verify=header_sender command. I also fixed some of the header syntax
checking to recognize some additional valid, and sometimes used, address
list formats.

http://www.donelan.com/exim-patches/header-verify.patch
http://www.donelan.com/exim-patches/header-verify.readme

verify = header_sender[=header_type]/<options>

Header type can specify which "sender" header field to check.

   *         returns true if any sender header address is verifiable
   all       returns true if all header sender addresses are verifiable
   from      returns true if verifiable address in From: header line
   reply_to  returns true if verifiable address in Reply-To: header line
   sender    returns true if verifiable address in Sender: header line

The default action is "*" which is the current (4.70) behavior.

Example from EXIM configuration

# Check if just the header From: can be (partially) verified

   warn    condition     = ${if def:h_from: {true}{false}}
           !verify       = header_sender=from
           logwrite      = X-Header-Error: $acl_verify_message
           add_header    = :at_start:X-Header-Error: \
                          could not verify "From:" header address