For communication over the internet different formats and standards are defined. There is a standard defined for internet based retrieval of emails by the users from the email server which is POP3 The number denotes that this is the third version of the protocol.
This protocol works at the Application layer of the reference model. It is used over a secure TCP/IP connection. POP3 is the most widely used protocol, IMAP is also used.
This protocol is now used with its Third version. You can see that his protocol has undergone several revisions and now it has been currently with all the needed features. The design of the POP protocol supports end users with the temporary internet connections and allows them to retrieve the mails when connected and then view them and manipulate the retrieved mails when offline
Clients that use the option to leave the mail on the server generally use the POP3 Unique Identification Listing (UIDL) command. Mostly the most of commands of the POP3 identify the messages by the ordinal number on the mail sever. This creates issues for a user to leave the mail on the server, as this numbers change from one connection to the server to other.UIDL helps in avoiding this confusion as it is assigns unique string of characters for every message. When the user connects to the server using the POP3 compatible email client it can use this UIDL command to get the current mappings from the messages to ordinal numbers. This is useful in recognizing which messages it has yet to download
POP3 contains a protocol extensions know as XTND XMIT that allows to send oitboud mails. In the previous versions POP used unencrypted login mechanism, currently it supports a well structured Authentication to provide a legitimate access to the users. The APOP uses the MD5 hash function to avoid replay attacks
POP uses TCP i.e. Transmission Control Protocol on port no 110. The transmission can be kept as encrypted using the transport Layer security or SSL (Secured socket layer). Some clients such as Gmail uses different port number that the normal ones that which is a deprecated alternate method which uses the TCP port no 995 .