POP

• The Post Office Protocol, now in its third rendition, is the protocol that email clients use to download email messages from the email server to the local computer. Once the email program downloads messages, the user can stop her Internet connection and read the messages from their location on her computer.
  
  

SMTP

• Simple Mail Transfer Protocol works in conjunction with POP3 to allow users to send emails from their local computers to other email users. The outgoing (sending) mail server uses this protocol to direct the message to the appropriate mail server so the recipient can then use POP to download the messages.
  
  

IMAP

• Internet Message Access Protocol is an alternative to POP3. The user can access individual or multiple email messages as well as create, edit or delete folders and mailboxes on the email server via his email client. However, this protocol leaves the messages on the email server; thus, the user must always be connected to the Internet to view messages.