A simple to implement method to help protect your domain from email spoofing is by setting up a Sender Policy Framework.
When an email is received from your domain, the system would have the ability to check the SPF record on your DNS to see if the email came from a trusted source.
For example for a google hosted domain name you would add a txt record containing:
v=spf1 include:_spf.google.com ~all
Note: ~all is not the same as -all
An example email header before a SPF record existed would look like:
Received: from mail-pw0-f46.google.com (mail-pw0-f46.google.com [209.85.160.46])
by mx.google.com with ESMTPS id c3si23440902ict.140.2011.06.21.14.25.35
(version=TLSv1/SSLv3 cipher=OTHER);
Tue, 21 Jun 2011 14:25:35 -0700 (PDT)
Received-SPF: neutral (google.com: 209.85.160.46 is neither permitted nor denied by best guess record for domain of emailaddress@nexidyne.com) client-ip=209.85.160.46;
Authentication-Results: mx.google.com; spf=neutral (google.com: 209.85.160.46 is neither permitted nor denied by best guess record for domain of emailaddress@nexidyne.com) smtp.mail=emailaddress@nexidyne.com
After a SPF record has been added the header would look like:
Received: from mail-iy0-f174.google.com (mail-iy0-f174.google.com [209.85.210.174])
by mx.google.com with ESMTPS id bi3si3658024icb.148.2012.02.15.12.56.14
(version=TLSv1/SSLv3 cipher=OTHER);
Wed, 15 Feb 2012 12:56:15 -0800 (PST)
Received-SPF: pass (google.com: domain of emailaddress@nexidyne.com designates 209.85.210.174 as permitted sender) client-ip=209.85.210.174;
Authentication-Results: mx.google.com; spf=pass (google.com: domain of emailaddress@nexidyne.com designates 209.85.210.174 as permitted sender) smtp.mail=emailaddress@nexidyne.com
For other email servers, for example Dreamhost (http://wiki.dreamhost.com/SPF) you can add something like:
v=spf1 ip4:64.111.100.0/24 ip4:66.33.201.0/24 ip4:66.33.216.0/24 ip4:208.97.132.0/24 ip4:208.97.187.0/24 ip4:208.113.200.0/24 ip4:208.113.244.0/24 ip4:208.113.175.0/24 mx -all
You can check to see if you have this record by issuing the *nix command nslookup -query=any domainname.ext