The solution offered did not match my situation, so I had to modify it a bit. The first problem was that the original script parses only /etc/passwd and the aliasfiles of Postfix itself. I also have virtual aliases and multiple domains, so I modified it to parse those too, you can download it here (parse_relays.pl.gz). At that moment I was thinking about how to automate it further.

The original script uses scp to transfer a generated file and the database generated from that to the secondary mailserver. It checked if there were any changes before doing that though, this meant that if I were to automate that, it would be with a crontab and thus polling. Luckily there are more efficient methods in Linux: inotify.

Inotify is a kernel-subsystem which can watch files for events, like opening, writing and closing and it does this without polling! I installed inotify-tools and use the inotifywait tool to watch a couple of files, if they are altered the shellscript continues running and will parse the files and send the result to the secondary mailserver, where a new database will be generated.

#!/bin/sh
 
FILES="/etc/passwd /etc/aliases.db /etc/postfix/aliases.db \
        /etc/postfix/virtual.db /etc/postfix/domains"
 
case "$1" in
    start)
        echo "Starting Postfix Relay updater."
        /root/bin/update_relays.sh &
        exit
        ;;
    stop)
        exit
        ;;
esac
 
while true; do
    # Wait for filemodifications
    inotifywait -qqe MODIFY ${FILES}
 
    # Sleep half a second to prevent bashing
    sleep 0.5
 
    # Send the file to the secondary mailserver
    /root/bin/parse_relays.pl |
        ssh -qTi ~/.ssh/postfix_relay user@mail2.example.org
done

To automate the transfer to the secondary mailserver, I generated a ssh-key on the primary (ssh-keyget -t dsa -f /path/to/key). The public-key part of this key should be transfered to the secondary mailserver and placed in ~/.ssh/authorized_keys, prepended by something like from="1.2.3.4",command="/home/user/bin/receive_relays.sh",no-port-forwarding,no-X11-forwarding,no-agent-forwarding,no-pty.

receive_relays.sh is executed at the moment an ssh-connection comes in which authenticates itself with the generated key. It places everything from stdin in ~/.relay/relay_example_org and runs postmap on it.

#!/bin/sh
 
RELAY_FILE="/home/user/.relay/relay_example_org"
 
# Update the file
cat >${RELAY_FILE}
 
# Create the Postfix database
/usr/sbin/postmap ${RELAY_FILE}

If Postfix is configured correctly (see the part about relay_recipient_maps in above article) all should be well and completely automated!