The explanation of SendMail Server forwarding function under Linux

1, why can not configure the mail server for open relay?

If your system administrator sets your mail server to open relay, will cause some spammers to use your mail server as a relay station for forwarding spam messages, which will cause spammers to target you, potentially causing retaliatory mail bombs; spam can also consume a lot of your resources, Occupy your bandwidth. The worse thing may be that your name may be blacklisted and become the target of a boycott by other recipients, and your email will be rejected by these recipients.

2, what is mail forwarding (mail relay)

Set up an email server, the server will have one or several domain names, when the email server will listen to Port 25th, waiting for Remote Mail request. Other mail servers on the network, or MUA (mail User agents, such as Outlook Express, Foxmail, and so on) that request to send mail will connect to Port 25th on the email server and request a message to be sent. The SMTP session process typically starts with the remote identity itself, as follows:　　　　　　　 HELO remote.system.domainname
　　　　　　　 250 qmailserver.domain
　　　　　　　 MAIL FROM：user@somewherer.net
　　　　　　 　250 OK
　　　　　　　 RCPT TO: user1@elsewhere.net

The domain name in the recipient user1@elsewhere.net of the message is not necessarily a local domain name, and the local system may have two answers that accept it:　　　　　　250 OK

or refuse to accept it:　　　　　553 sorry,.that domain isnot in my domain list of allowed recphosts

In the first case, the local email server is allowed to relay, it receives and agrees to deliver a message with a destination address that is not a local domain name, and the second does not receive a non-local message.

Email usually has a configuration file that determines whether to accept a message. The message is accepted only if the domain name of the recipient address in a RCPT TO command is present in the file, or the message is rejected. If the file does not exist, all messages will be accepted. When a mail server forwards (relay) all messages, regardless of the recipient of the message and the recipient of the message, the mail server is referred to as open forwarding (relay). When the email server does not set forwarding restrictions, it is open forwarding.

3, sendmail How to limit mail forwarding

Because send mail in order to be afraid to become the advertising letter to the letter station, so after installing only the line on the computer users to send a letter, you need to make it for you to change something. Modify the/etc/mail/access file to add the following lines to the inside.　　localhost　　　　RELAY
　　192.168.1　　　　RELAY

You can turn the letter in the 192.168.1 domain.