Here's a techie tip that will be of interest if you run your own email server. To be fair, most people don't run a mail server - they use the one provided by their company, or they use systems such as Gmail. But some companies and enthusiasts like to operate their own mail server. If you're such an enthusiast, the golden rule is that you must ensure your server isn't an "open relay".
In a nutshell, an email server that is misconfigured as an open relay can allow spammers to use it for sending unwanted email to just about anyone. Which will quickly results in your email server becoming blacklisted, and then you'll discover that none of your messages ever gets through to its intended destination.
One useful site for checking that your mail server is playing by the rules, is Mx Toolbox. For example, head to http://mxtoolbox.com/diagnostic.aspx and type the IP address of your mail server into the box, and the system will attempt to connect to it as if it were an open relay. You'll then get a report as to whether or not the tests were successful.
If you provide IT support to a company that runs its own mail server, you really owe it to yourself and your client to run this test as soon as possible.