Email services are regularly receiving phishing attacks. Our servers block many but a few get by on occasion.
A Phishing attack is usually an official sounding email about a bank, ebay, paypal, facebook, etc that is looking to get you to follow a link to fill in information about yourself or account information so that your account isn’t closed, a fee isn’t charged to your credit card, or something else that gives us a worried emotional response. Best thing to do with these emails is to delete them. Occasionally they come with a message to open a zip file. Our email server strips zip files as they are a traditional way to send a virus to unsuspecting users.
For more info for you or your staff have a look at the wikipedia article on phishing http://en.wikipedia.org/wiki/Phishing