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To transition sending email to a new ip or range

Send a certain percent of messages on the new and old ip. To do this on one machine which has both ips, add these to the remote smtp transports. For example, in /etc/exim4/conf.d/transports.conf depending on the config setup.

helo_data = ${lookup dnsdb {ptr=$sending_ip_address}{$value}{$primary_hostname}}
interface = ${if < {${randint:1000}}{3} {209.51.188.17}{208.118.235.17}}

In this example the new ip is 209.51.188.17 and the old is 208.118.235.17 and this makes it send 3/1000 messages on the new ip.

Progression should be slowly increasing. We used this daily sequence change in the 2018/2019 change: 5 10 15 20 30 40 60 80 120 160 220 320 480 640 860 1000

For exim about 4.71 or earlier, the rng is bad or nonexistent, so use this instead.

interface = ${if <= {${run{/usr/bin/python -c "import random; import sys; sys.stdout.write(str(random.randint(1,1000)))"}}}{5} {209.51.188.13}{208.118.235.13}}

For testing on the normal mail servers, i did this config and test message:

interface = ${if eq {iankelling.org}{$domain} {209.51.188.13}{208.118.235.13}}
echo "test body" | mail -s "test mail from $HOSTNAME" ian@iankelling.org

and to verify the helo, i ran this

   exim -d -t <<'EOF'
From: root@gnu.org
To: ian@iankelling.org
Subject: Testing Exim

This is a test message.
EOF

For testing config changes, run update-exim4.conf, which will affect subsequent runs of exim, for example when you send a test message, or when new queue runners are spawned, but it won't restart existing processes, thus it will have a less detrimental affect if things are wrong. There are better ways to test new configs where the new queue runner's won't pick it up, but I haven't figured it out yet. After a change is tested, restart exim.