TIdPOP3 is a TIdMessageClient descendant that provides a client implementation of the Post Office Protocol version 3 (POP3), as described in the Internet Standards documents: 

RFC 1939 - Post Office Protocol Version 3  

RFC 1734 - POP3 AUTHentication command  

RFC 1957 - Some Observations on Implementations of the Post Office Protocol (POP3)  

TIdPOP3 is used to retrieve E-Mail messages on a mail server and return the messages to the user's computer. TIdPOP3 provides methods and properties that allow a client account to connect to a POP3 server, and to retreive various information about the size and contents of a mail box or email message. 

TIdPOP3 is not intended to provide extensive manipulation operations of mail on the server; normally, mail is downloaded and then deleted. Use TIdIMAP4 and TIdIMAP4Server when a more advanced (and complex) mail protocol is required. 

TIdMessageClient 

TIdIMAP4 

IndySupport