POP3 stands for “Post Office Protocol 3”. It’s a simple protocol with limited functionality. It’s function is to retrieve and delete an email from the POP3 server. POP3 email protocol is installed on both ends. I mean on client’s computer as well as on mail server.
How does POP3 work?
POP3 starts working when a user connects with the mail server using port number 110. POP3 server basically works in three steps
In the first step user sends “username” and “password” for authentication to get access to his/her mailbox.
In this step user can receive messages as well as mark messages for deletion. In simple words the user tells the POP3 mail server either to keep an email or remove it from mailbox.
This step starts working when user log off or give quit command. After logging off, the mail server takes appropriate actions performed in step 2.
There are two possible replies (+OK or -ERR.) from the POP3 server when user give any command. For example if you type your username or password incorrect. The server will reply with -ERR command. If you type correct spellings of your username and password then server will respond with +OK.
These days usually users use mail services on their PC as well as on mobile phones. In that case POP3 works differently. I mean if you are using POP3 service on multiple machines.
For example you download and read a messages on your PC. Then POP3 server will not show this as unread message on your mobile device if you later access mail server from mobile device.
Similarly if you download and then delete an email using your mobile device. Then later you cannot reread that email using your PC.
Remember POP3 is an application layer protocol of OSI model