How does one receive an email which is post dated?

Recently I signed up with some temp agencies online and also browse around some sites online and wrote down my email address for some free gift certificates.





The problem - now I am receiving 10 to 20 emails a day. My bulk inbox is over 5,000.





But I also receive some post dated emails. ex. I got some emails with date sent of 6/27/06. Today is only 6/21/06.





Any explanations will be appreciated. Please touch a little on security issue also because I like to protect myself. However, I really do need a job and most jobs require online information.

How does one receive an email which is post dated?
If you write a program to send the email you can change all aspects of the message, including date/times, sender, etc...