What is a POP3 Server?
By Matt Western.

POP3 is an acronym derived from the name of an email protocol. POP stands for Post Office Protocol and the 3 stands for version or revision 3.

POP is a protocol used by email client software such as Outlook, Outlook Express, Eudora, Thunderbird etc to access mail in an email mailbox.

As I mentioned, POP3 is a protocol. A protocol is a just a set of rules that have been designed to perform and complete a task and POP3 is an email protocol used to retrieve mail from an email server.

Just about every Internet service provider provides an email server that supports the POP3 protocol.

Prior to the POP3 standard, there were earlier versions, POP1 and POP2. Both these earlier versions are now obsolete due to the reliability and functionality of the POP3 version.

POP3 allows you log into your email server and download your email for reading offline. The POP3 protocol actually oversees the transfer of your email from your email server to your PC. Once the transfer is complete, your email mailbox on your email server is emptied.

However many email clients (Outlook, Outlook Express, Eudora, Thunderbird etc.) provide a function whereby you can elect to leave your email on your email server. In this case, a copy of your email is made and that copy is downloaded.

This can be useful when more than one person needs access to the same email as others. For example, email may be sent to a team of workers as a way of distributing internal notices.

An easy way to do this is to set up a single email address and allow the members of that team to download any email sent to that email address.

This is when it is useful to configure your email client software to leave a copy of all email on the server. As each team member logs in to download their email messages, the messages are retained on the server. In tis way, every team member will receive the email.

In day to day use, you may hear of email servers being referred to as POP servers. This is just a shot way of saying POP3. Although the word POP is used, it will in 99.99% of cases in fact be a POP3 server.

POP3 servers are fairly simple and consist of few commands. Here are the most common:

- USER - enter your user ID
- PASS - enter your password
- QUIT - quit the POP3 server
- LIST - list the messages and their size
- RETR - retrieve a message, pass it a message number
- DELE - delete a message, pass it a message number
- TOP - show the top x lines of a message, pass it a message number and the number of lines

POP3 servers maintain the email they retrieve as a list of text files. These text files, one for each email account of the POP3 server are the users email messages. When a new message arrives the POP3 server adds the new message to bottom of the recipients text file.

POP3 servers require that you supply an account name and password and once they are supplied and verified the client software (Outlook, Outlook Express, Eudora, Thunderbird etc.) is then able to download the recipients' list of messages (text file).

POP3 servers operate on TCP port 110.