Their mistakes will catch up to them in the long run. Often times, less is more in terms of the message itself - keep it as brief as possible. Also, don't convey sensitive messages in an e-mail. E-mail and voice mail can be easily saved and routed to other people, so ask yourself if you'd be comfortable with your message landing in other people's inboxes. Also, don't be afraid to ask people about their communication preferences.
Most people appreciate being asked how they like to receive memos, updates, urgent requests, etc.
Check with your boss and colleagues on this question so you can establish and follow the proper protocol. If this person works outside your department, you may wish to let his manager know. Check OfficeTeam's Workplace Protocol Quiz for more advice. That's what matters most in terms of protocol - managers and other people who are in a position to promote you and move your career forward will notice that you're taking the high road. That could change the way your coworkers respond to you as well.