(1) Simple Mail Transfer Protocol (SMTP)
(2) Post Office Protocol (POP)
Email is widely use Internet features. So, anyone uses it for malicious purpose with variety basis like:
(1) Abusive Email
(2) Email Forging
(3) Spam Email
Out of above, Email Forging is widely use for malicious purpose. It allows anyone to disguise the source of an Email and send it to the victim. It is the art of sending an Email from the victim's Email account without knowing the password.
For example: If you know your friend's Email ID then you can Email and inform his girl friend that he hates her and will break up them relationship. Or, If you know your Principle's Email ID then you can Email and inform your entire college that it will be holiday on tomorrow. Or, If you know your Boss/CEO's Email ID then you can Email on behalf of your CEO and inform entire office staff whatever you want. (all these are malicious activities so I myself don't allow)
But you can use it for Fun and just fool your friend using this tricks. Just fill out following Form and send Email. (I am not liable for anything so be careful while using this trick)