Message transfer Agent sendmail-basic Configuration certificate Mail encryption authentication

SendMail is the most important message transfer agent. SendMail as a free mail server software, has been widely used in a variety of servers, it in the stability, portability, and ensure that there is no bug has a certain characteristics, and can be found in the network a large number of use data. In general, we decompose email programs into user agents, transport agents, and delivery agents. The user agent is used to accept the user's instructions and send the user's letters to the letter transfer agent. The delivery agent obtains the letter from the letter transfer agent to the end user's mailbox. The message transfer is from the server to the server, and each user must have the space (called the mailbox) where the information is stored on the server to accept the message (the message is not sent by this restriction). As you can see, the primary job of a message transfer agent is to monitor the user agent's request, locate the corresponding mail server based on the destination address of the e-mail message, transfer the mail between the servers, and buffer or submit the received message to the final delivery program. There are a number of programs that can act as message transfer agents, but SendMail is one of the most important, and it turns out that it can support thousands of or even more users, and that it takes up very little system resources. However, the configuration of SendMail is very complex, and no authentication, no garbage prevention mechanism, no anti-virus mechanism. When the SendMail program gets a message to be sent, it needs to determine the delivery of the letter to the corresponding server based on the destination address, which is implemented through the DNS service. SendMail first determines that the address is a user name + machine name format, and then queries DNS to determine the need to post a letter to a server. In DNS data, an MX record is associated with an e-mail message.

MUA (mail user agent) for sending or reading messages on the client side. Linux has mutt this tool MTA (mail tranfer Agent message transfer agent), equivalent to a post office, server-side software, the main function is to receive MUA mail and send mail to the next MTA, which can be said to be a message route (mail Router), server-side software belongs to the MTA, now open source has sendmail,postfix,qmail and so on.

The MDA (mail devilery agent mail delivery Agent), which mainly refers to messages accepted by the MTA, which are placed under a local account at the destination of the message, or to the next MTA, generally to a command such as mail.

Protocol for Mail:

Letter: SMTP (Simple Mail Tranfer Protocol) port number TCP 25 port, when the letter, MUA will actively connect the MTA port 25, and then sent through the SMTP protocol, SMTP is divided into accept SMTP and send SM TP, which regardless of the host's configuration or system, as long as the two sides of the SMTP protocol OK can send mail

Receipt: POP (Post Office Protocol Post-Office Protocol) to connect to the MTA to read or download messages, now the most commonly used version is POP3, the port is the Internet Message Access Protocol network File agreement), can download the message before downloading mail header information, so that users can selectively download port 143.

First, test the environment.

Here I use the CentOS system, which by default seems to have been installed SendMail, and has been started.

Then install Dovecot:yum install Dovecot

Start Dovecot:service dovecot Start

At this point can send a letter, in order to test can add two account user1, User2, and then send a message to it

See by the log, send success, really send success, to see User1 can receive this title 1 content of 12387681498649821 of the message, in order to verify, to switch to User1, carry out the letter