older article that I’ve revived.

Problem⌗

The problem: you have a laptop and you’re not always connected to the Internet. Still you want to sent mail even when you’re offline. You cannot use just any mail server out there, ‘cause a lot of them don’t relay. So you must use your own mail server.

You'll need:

1. postfix, only used for queuing and forwarding the mail
2. openSSH, for setting up a tunnel

OpenSSH config You will need to create a ssh tunnel to your mail server. This is the command I use:

ssh -2 -N -f -L 10025:elektron.atoom.net:25 miekg@elektron.atoom.net 2>/dev/null

Of course you also want to setup ssh so that you can login without typing a password.

postfix config Next you must tell postfix to use you’re tunnel. It is also important to keep postfix from doing MX lookups. In the main.cf of postfix add the following:

relayhost = [127.0.0.1]:10025   # use the tunnel, no MX lookups
defer_transports = smtp               # only send when online
mydestination = localhost.localdomain   # not sure if this is needed

system config Put the startup commands in /etc/dhclient-exit-hooks. This way every time you get a IP number from a dhcp-server the tunnel is re-established and the mail is flushed, /etc/dhclient-exit-hooks:

# start an ssh tunnel to elektron
ssh -2 -N -f -L 10025:elektron.atoom.net:25 miekg@elektron.atoom.net 2>/dev/null

# run the mailqueue
/usr/sbin/sendmail -q

Also make a cronjob that runs every now and then to flush the queue:

# run queue every 5 minutes
*/5 * * * *     root  test -x /usr/sbin/sendmail && \ 
nice -n10 /usr/sbin/sendmail -q

That’s it. Mail should now be queued until it can be delivered via you’re own mail server.