Start out by flashing the Soekris Net5501 with the latest firmware. Can also be downloaded here.
On how to flash it look here or a local copy here.
This put the console speed up to 57600 and locks it from change by applications.
As the Soekris has limited input options I used PXE boot to start the installation of Hardy Heron. Of course this requires an other machine to service a tftp server and a dhcp server. You can find inspiration on how to do that here. For unknown reasons the console display is unusable if you use the pxelinux.0 file that comes with Ubuntu. Therefore use this one instead.
As the console speed is now 57.6Kbps, this needs to be changed in the PXE boot config file. Edit the file /var/lib/tftpboot/ubuntu-installer/i386/pxelinux.cfg.serial-9600/default and change all occurrences of 9600 to 57600 then save in its own directory of /var/lib/tftpboot/ubuntu-installer/i386/pxelinux.cfg.serial-57600/ and remember to change the symlink pxelinux.cfg in /var/lib/tftpboot to point to the new 57600 directory.
Go through the install like an ordinary Ubuntu server installation except when you get to the partitioning. The boot partition has to be within the first 1023 cylinders otherwise Grub doesn't handle it so well (you'll get an GRUB ERROR 18). I believe it has to do with Grub not being able to reading the boot files. I made the /boot partition around 100MB of size and I was home free.
A small note, when getting to keyboard layout, autodetect doesn't work, just select it manually.
Pick you partition layout. Make /boot within the first 1023 blocks (make it 100MB ex.), hit install and wait as the packages are downloaded from the Internet and installed.
When you get to the “Software selection” menu I select “LAMP server” and “OpenSSH server”. You may do otherwise but this gets the needed software in quick.
With this info at hand you can copy this main.cf file and replace the corresponding postfix uid/guid with your values.
For this install I chose to use dovecot instead of couirer-imapd. And it turned out to be a good choice with easy integration for Postfix SASL.