POP, also known as Post Office Protocol is an application-layer Internet standard protocol that is used by email clients to retrieve e-mail from a mail server.
This standard does not include the means to send email messages therefore you need to use the Simple Mail Transfer Protocol (SMTP) to send emails from the client.
The current version is POP version 3 (POP3) published in 1988. POP1 was published in 1984 while POP2 was published in 1985.
How POP Internet Protocol Works
Incoming email messages are stored on a mail server that supports the POP standard. Once the user logs in with an email client, the client proceeds to download the available messages in an insecure manner over port 110 and stores them on their computer.
- STARTTLS: This extension adds security and allows the use of Transport Layer Security (TLS) or Secure Sockets Layer (SSL) to be negotiated using the STLS command, on the standard POP3 port. It is the convention for most email clients and servers to use the alternate-port method, which defaults to TCP port 995 (POP3S).
- SDPS (Standard Dial-up POP3 Service): This extension allows multiple accounts per domain. To access each account, the username includes the hostname, as [email protected] or user+hostname.
- Kerberized Post Office Protocol (KPOP): With this extension, local e-mail clients can use the Kerberized Post Office Protocol (KPOP), to retrieve e-mail from a remote server over a TCP/IP connection. The KPOP protocol is based on the POP3 protocol but it adds Kerberos security and that it runs by default over TCP port number 1109 instead of 110.
POP is a limited protocol that allows an email program to only download messages to the computer or device, with an option to keep a copy on the server for future download.
Where copies are left on the server, POP allows email clients to track retrieved messages, but quite often this feature fails for varied reasons resulting in messages download again causing duplicates.
This protocol does not support the access of the same email account from multiple computers or devices and has actions to synchronize between them.