How do email clients retrieve messages from server?

You see, there are Protocols called HTTP and POP.

HTTP stands for Hyper Text Transfer Protocol

while,

POP stands for Post Office Protocol.

These two sets of Rules enable server to identify that who sent the email to whom. HTTP is responsible to maintain the format of the Email, while POP helps server

1. to identify which computers users' email is it
2. Is he online checking the email
3. has HTTP contacted server a possible interaction from user requesting email access