I have just installed Domain Key Identified Mail (DKIM)  on my mail server. In summary the purpose is to allow public-key signing of all mail that goes out from your domain so that the recipient can verify its authenticity (and optionally reject forgeries). It also means that you can verify inbound mail.
The first thing to do before installing DKIM is to get a Gmail account. Gmail gives free accounts and does DKIM checks. If you use Iceweasel or another well supported browser then you can click on Show Details from the message view which then gives fields mailed-by and signed-by which indicate the DKIM status. If you use a less supported browser such as Konqueror then you have to click on the Show Original link to see the headers and inspect the DKIM status there (you want to see dkim=pass in the Authentication-Results header). Also Gmail signs outbound mail so it can be used to test verification of received mail.