The Post Office Protocol (POP) is a transfer protocol that allows a client to retrieve e-mails from an e-mail server. Version 3 (POP3) is described with RFC 1939. POP3 is an ASCII protocol in which the data transfer is controlled by means of commands that are sent to Port 110 as default. POP3 has very restricted functionality and enables only the listing, retrieval and deleting of e-mails on the e-mail server. For other functionalities, such as hierarchical mailboxes directly on the mail server, access to several mailboxes during a session, pre-selection of e-mails, etc., protocols like IMAP must be used. As a counterpart to POP3, the Simple Mail Transfer Protocol (SMTP) is commonly implemented by clients and servers for sending e-mails.