IMAP/POP3
Internet Message Access Protocol is a message retrieval system common in most mail servers and email clients. It supports both offline and online operation modes and functions by transferring messages from a server to a client. IMAP does not automatically deletes messages from the server even after downloading them, unless manually done by the user.
Post Office Protocol is another system that retrieves mail from a server using TCP/IP connection and among the two most popular (the other being the IMAP) as most webmail service providers (Yahoo, GMail, etc) support them. The POP3 version is now the prevalent standard that uses port 110 and encrypts connection via STLS. Users have a choice to leave the email messages on server after download but most email clients connecting via POP will delete them from the server once the retrieval is done and the connection is cut.