POP server

When X connects to the Internet the next time and wants to see the new emails that have arrived for him since the last time he had connected to the Internet, he opens his email client program. That email client program on his computer in turn invokes a POP client, which contacts the POP server hosted by the ISP. The POP server then opens the mailbox for user X and sends the emails arrived for him to the POP client (i.e. to the user's computer).

In simple terms, any user who wants to receive emails but does not have a permanent connection to the Internet uses a POP client to pull emails from a POP server. The POP server consults the user's mailbox to perform this task. In the case of a company that provides an email access to all its employees, the SMTP and POP servers have be hosted by the company itself, instead of the ISP.

Why is POP needed? We know that emails are stored on a centralised email server SMTP server. SMTP server expects the destination host (i.e. the email recipient) to be online all the time. Without this, it cannot create a TCP connection with it, and therefore, cannot forward the email message to the destination host. As we know, desktop computers are usually powered down when the business hours are over. Therefore, the solution to this problem is having a POP server. Whereas the SMTP server in an organization receives and stores all the incoming emails for any users in that organization, it is not used for transporting the emails to the end destination. Instead, the SMTP server forwards the incoming emails to the user's mailbox, and the POP server retrieves the emails from the respective email boxes of the users when requested by the POP clients. When a user's POP client connects to the POP server, the POP server opens the mailbox for that user and sends the new emails to that user.

In other words, it should not be expected that for a user to access his emails, he should either be connected to the Internet all the time, or he would have to physically go where the SMTP server is kept and access his emails. To avoid this situation, the TCP/IP protocol suite includes a protocol: POP, which allows a user's mailbox to be remotely accessed from the email server from the user's computer itself. The protocol provides a facility for a user's mailbox to reside on the email server and for accessing items in that mailbox from the user's computer using a connection between the user's computer (the client) and the email server.

Of course, the user's email software is responsible for connecting to the POP server and for requesting unread mails in the first place. Based on this request, the POP server performs the operations described earlier.

For forwarding an email message to the actual recipient, a POP server simply maintains a collection of text files, one per user. The text file contains all the email messages for that user, one after the other. Therefore, when an email arrives for a user, it simply appends the email at the end of the text file for that user. This text file for a user is called mailbox. When a user (client) connects to the POP server, the POP server looks at the user name and then opens the corresponding text file for that user (which contains new and therefore, unread emails for that user). It then sends the contents of this text file which are actually new email messages to that user. The current version of POP is 3, and therefore, it is sometimes also known as POP3.

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