奥克兰论文代写-邮件发送过程的分析。假设用户需要向电子邮件地址为“firstname.lastname@example.org”的另一个用户发送一封信，那么他或她应该首先向主机mail.com发送一封信。由于无法通过网络将这封信“直接”发送到主机mail.com，因此用户必须设置邮件服务器才能投递。用户应该注册到邮件服务器，以获得能够发送邮件的合法电子邮件权限。换句话说，用户应该利用界面发送电子邮件。下面是说明。邮件用户代理:如前所述，用户使用telnet等软件登录邮件服务器来发送信件。它们必须依赖于邮件服务器的MUA或邮件用户代理。常见的MUAs包括Mozilla推出的雷鸟(Thunderbird)免费软件、Linux桌面KDE通用Kmail、Windows Outlook Outlook Express (OE)等。MUA的主要功能是接收主机电子邮件，使用户能够浏览和编写电子邮件功能。
MTA, also Mail Transfer Agent, can be regarded as a post office. After MUA send mails to the mail server, the mail server is then responsible for the following delivery. The mail server is called “MTA”. MTA has following two functions:
1. Receive mail by using Simple Mail Transfer Protocol through Port 25
2. Relay mail. If a mail’s destination is not the mail server’s user and data related to this mail has clearance to use MTA, then MTA will send this to other mail servers, which is called Relay.
Although strict MTA only refers to Simple Mail Transfer Protocol, mail server is still recognized as “MTA”. Those software featuring SMTP functions are Sendmail, postfix, Qmail, etc.
MDA is the abbreviation for Mail Delivery Agent. In fact, MDA is a subordination of MTA. If MTA is the post office, then MDA is the postman. MDA exams the prefix or content of the mail and decides where this mail should go. The relay function mentioned above is done by MDA. MDA also has the function to filter junk mail and reply automatically. That software mentioned in the last paragraph has their own MDA which have more powerful functions.
The last concept is the mailbox where incoming mails are processed. In Linux, the default mailbox is in /var/spool/mail/user’s email address.
It is important to understand how MUA send the user’s mails all the way to the receiver’s mailbox.
1. If we want to use local MUA to use MTA to send a mail, we firstly need to gain access to a MTA. And that is what we actually do when we register an E-mail account and set password.
2. We edit an E-mail which includes E-mail addresses of the sender and receiver, and subject. Then we add contents. After editing, we press the send button, and the mail is now sent to a MTA. Notice that the mail is sent to the sender’s MTA instead of the receiver`s. If the sender has access to MTA, then the mail will be in queue to wait to be sent out.
3. If users are sending mails to themselves, then mails will be directly sent to the mailbox. If users are sending to others, MTA will analyze to make sure users have legal access to send it. If users do, MDA will transfer it to another MTA through SMTP port 25. If the mail is successfully transferred, it will be removed from the queue.
4. Then the receiver’s MTA will receive senders’ mails and put it into mailbox correctly. In this process, MDA will decide whether it is a junk mail.
I decided to choose mail server as my project topic because I am personally interested in the practical application of mail servers and plan to design my own mail server at home. In addition, despite the growing popularity of email in the modern communication, the general public often pay limited attention to the function of a mail server. Maintenance, security and troubleshooting are all time-consuming tasks that call upon further attention. Therefore, this project is designed to give an inside view of the construction and operation of mail servers.
In next chapter three different Mail Transfer Agents (MTA) which handle MTA protocols, namely, Postfix, Mailman Sendmail, are selected for analysis.