#! /bin/sh
#  (Bourne shell script) -- reports on mail exchangers for a domain
#
# The command "host -t mx example.com" can give no response (a blank line)
# if there are no MX records, or the following:
#   moo.com mail is handled by 6 us-mx.moo.com.
#   moo.com mail is handled by 5 mx.moo.com.

# Note: this command is partly redundant, given the following example:
#   $ host ausmedsupply.com.au
#   ausmedsupply.com.au has address 203.82.212.50
#   ausmedsupply.com.au mail is handled by 50 email.webvault.com.au.
#   ausmedsupply.com.au mail is handled by 100 mail2.webvault.com.au.

# TO-DO: catch and report the error filtered out by the grep
#        (first two are fatal and indicate domain problems; 3rd one means that
#        fqdn handles its own mail)

set -e
host -t mx "$@" |
  grep -v -e 'NXDOMAIN' -e 'connection timed out' -e 'has no MX record' |
  sort -k6n |
  while read a b c d e n mx blah ; do
    host=$(host $mx | head -n 1)
    echo "$host (priority $n)"
    set $host
    # If result is an IP address, reverse-resolve it
    if expr "$4" : '^[0-9]' > /dev/null
    then
      echo "$(host $4)" | sed 's/^.* \([^ ]*\)\.$/  \1/'
    fi
  done