Mail servers that are used for processing email are very simple to understand. They work diligently in the background in order to route the emails that are sent from the sender to the recipient. This is how they are designed to work (in the simplest sense):
The basics of POP and IMAP
The need to have an email client on the machine that will be accessing the internet is essential. There are two basic types of email client Post Office Protocol (POP) and Internet Message Access Protocol (IMAP). Each is processed differently in terms of mail server actions.
With the email client in place, the mail server will need to be able to create a connection and be accessed by the client. In POP mail clients, the client will send a request to the server to access to account and download the mail to the email client on the device (such as a computer or mobile phone). The mail is saved to the device and can be accessed even after the internet connection is lost because it is stored locally. In IMAP mail clients, the client will access the server but the server keeps the mail on the server. This makes it possible for users to access their mail anywhere that they can gain internet access with their IMAP client enabled device. Usually IMAP clients can be accessed through web browsers and mobile devices.
When sending mail to the server and ultimately to the recipient, both POP and IMAP use the Simple Mail Transfer Protocol (SMTP). SMTP basically sends the mail to a route which will reach its destination. SMTP is used for sending messages only and is very efficient at forwarding the messages to their final destination. When the destination cannot be reached, SMTP will alert the server and the server will then use SMTP to send a message back to the original sender saying that the mail was not deliverable.
This is where the differences between free and paid email services are different for the users of the services. Most free email services are very extremely limited in what they can send and receive. They will usually be designed to work in IMAP to make it simpler for the accessing users to have access. A paid email service will more than likely use POP services while some have been observed to use IMAP as well as a hybrid of these types as well. Free options are more likely to have fewer options in usernames because they are accessible by everyone. Paid options will be more likely to allow access to usernames which are closer to your real name because fewer people have access to the service.
Mail servers perform a large amount of tasks that are usually not possible by people. Hundreds of thousands of messages are sent every minute and are managed through these servers and efficiently made to reach their destination. Users of such mail servers can really take advantage of what they have to offer when they understand what goes on behind the scenes.