Yes, laws relating to defamation are applicable to email and other online activities. For example, if a person commits libel against you through email or other online activities, the publisher, and any republisher, of the offensive statement can be held accountable for damages. This is why it is wise to be careful about anything you write in an email message or online chat room. If the victim is harmed by your action, you can be held liable for his or her losses.
If you think you've been defamed by false information passed on by a computer channel, or worry about whether you can make an aggressive advertising claim, it might be worth talking to a qualified lawyer.