Prerequisite : Sun J2SE 1.5 or 1.6. Tray icon is now implemented with SWT and compatible with Java 1.5. You may use DavMail with an older version, but the gateway will run as a command line application.
You should first download and install Java, e.g. by running the following command on Ubuntu :
sudo apt-get install sun-java6-bin
Download the linux x86 DavMail package from Sourceforge and uncompress it with your favorite tool. The standard package will run natively on x86, to use DavMail on any other hardware platform, replace the SWT with the right one from http://www.eclipse.org/swt/ or use the platform independent package. Sorry, no deb or rpm package yet !
On Ubuntu and other Gnome or Kde distributions, just use the desktop launcher.
On other distributions, try
davmail.sh.
You should now see the DavMail gateway icon in the tray :
Adjust DavMail settings : Getting started