google_outbound_smtp

#!/bin/sh

# extract Google's published list of outbound-going smtp servers, for purposes of whitelisting them
# from https://support.google.com/a/answer/60764?hl=en
# the results come back in CIDR notation, so be sure to handle that
# subtlety: i="" on the first iteration, because there is (currently) no _netblocks1, there's just _netblocks
(for i in "" `jot 3 1 3`; do
  dig TXT _netblocks$i.google.com @8.8.8.8 +short;
done) |
awk -F ":" -v RS=" " '/^ip4:/ { print $2 }'

# NB: if you're on linux replace `jot 3 1 3` with `seq 1 3`

# See also
# https://marc.info/?l=openbsd-misc&m=150679676125582&w=2
# http://web.britvault.co.uk/products/ungrey-robins/
# https://mail.live.com/mail/ipspace.aspx -> for hotmail
# https://answers.yahoo.com/question/index?qid=20100317121021AAog2Vp -> for yahoo, though with an unsatisfying result