Email is one of the most popular applications in the Internet. Just like the tradition mail delivery by the postman, email is also nonsynchronous, that is, people send and read the email when it is convenient without prior coordination with others. What is different from the traditional mail is that email is rapid and convenient in sending at a low cost. Besides, modern email may include hyperlink, HTML format text, image, voice and even video data. In the current office environment, email system has almost become a necessary tool.

Mailing service system mainly consists of three parts: user proxy, mail server and simple mail transfer protocol (SMTP).

We will explain every part in the following way: sender Alice sends an email to the receiver Bob. User proxy allows the user to read, reply, transfer, store and edit the mail (sometimes user proxy of email is called mail reader but we avoid this term herein). When Alice has written the email, her user proxy sends the information to the mail server, which puts the information in the queue to be sent. When Bob wants to read the email, his user proxy obtains the mail from the mailbox on the mail server. Mail server consists of following two types by the communication protocol:

SMTP server: it sends the mail and receives the external mail for the local user just like the mail reception department in the real life (which receives the mail sent by the ordinary user and other post offices)/

POP3/IMAP server: it assists the user in reading the mail for the user from the SMTP server.

Along with the popularization of email, the demand for email security is also increasing. We provide in our mail system proposal multilevel protection methods or measures against the rubbish mail, virus, malicious software, bot, phishing and bot attack.

Following is the typical topological diagram of mail system:

Case Study：
E-mail System Construction for a Financial Institution