[exim] Sender callout verification with warning only

We recently found that we could no longer use Exim's sender callout
verification on our MX because people here rely on various web services
etc. that send auto-generated messages "from" addresses with incorrect
local parts (and identifying all of them so that exceptions could be
made also seemed difficult.) But, then I thought that I might at least
add a warning if the callout fails, so the check_recipient ACL now has:

  require verify = sender
  warn    !verify = sender/callout=defer_ok
      message = X-Sender-Warning: The email setup at 
$sender_address_domain does not accept messages to $sender_address

Questions:

   1. Does this look correct to you? (Note that I still want to reject
      messages if the domain part is incorrect, hence the first test.)
   2. What header would you use for this warning?

--
Toralf