Due to the widespread use of web bugs in email, simply opening an email can potentially alert the sender that the address to which the email is sent is a valid address. This can also happen when the mail is 'reported' as spam, in some cases: if the email is forwarded for inspection, and opened, the sender will be notified in the same way as if the addressee opened it.
E-mail fraud may be avoided by:
Keeping one's personal e-mail address as secret as possible. Use a throw-away email when signing up websites online.
Using a spam filter. If you do not forward your ONID email, ONID will protect you from as much spam as possible with SpamAssassin. For help on setting this up, go here
Ignoring unsolicited e-mails of all types, simply deleting them.
Not giving in to greed, since greed is often the element that allows one to be "hooked".
Many frauds go unreported to authorities, due to shame, guilty feelings or embarrassment, but if you ever fall victim to an e-mail fraud that involves theft, either monetary or of your identity, contact the authorities immediately. You could help save many people from the same problem.