One would say that smaller businesses are more vulnerable since they may not have IT security measures or software like Sony so all of their information could be attained by a novice hacker. There are a couple of lessons from all of this.
1) Business emails should be all business. Because the line has blurred between our personal and professional lives we may get complacent and mix our content with our communication channels. Never add personal thoughts about others like coworkers, vendors, competitors or clients. Remember also that emails can be admissible in court for cases such as slander. In this case it's best to follow your mom's advice, 'if you can't say something nice about someone...'
2) Be selective when emailing. Remember, an email travels along a long path passing many different servers and equipment before reaching it's intended audience. Google, Microsoft and ISP's are always scanning emails for marketing opportunities so hackers may be able to as well. If a phone call or face to face would be appropriate for the information that needs to be relayed use those channels instead.
3) Have a policy or rules for emails. Setting up some rules about the appropriate use of email in a work setting is good especially if you have employees. If you are a sole proprietor or smaller organization make sure to keep your business and personal communications separate and be mindful of the information you transmit.
4) Most importantly, never ever click on links in emails that appear suspicious from unknown persons or vendors/clients. Hackers are getting more and more sophisticated with this process. They will send an email with malware that will collect your contacts and email them so your contacts believe they are getting an email from you when in reality they are getting an email from the hacker. Always check the address that it is coming from since this can be a dead give away (ex. the name display is John Smith from ABC Company but the address is firstname.lastname@example.org) As always, if you know the person and are not sure about the email you can call them to verify whether it is legitimate or not.
While it appears that anyone can be hacked, keeping business emails, business can help prevent alienating others around you, damaging your small businesses reputation or brand or opening your or your small business to slander and defamation.