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This article will explain in detail how to achieve the forwarding function of the Sendmail server under Linux. The editor thinks it is very practical, so I share it with you as a reference. I hope you can get something after reading this article.

1. Why can't I configure the mail server as open relay?

If the system administrator sets his mail server to open relay, it will cause some spammers to use your mail server as a relay station for forwarding self-emails, which will make spam recipients target you and may lead to retaliatory email bombs; spam can also consume a lot of your resources and take up your bandwidth. What may be worse is that your name may be blacklisted and become a target for other email recipients to boycott, and your email will be rejected by them.

2. What is mail forwarding (mail relay)

Once an email server is set up, the server will have one or more domain names, and the email server will listen on port 25, waiting for remote mail requests. Other mail servers on the network or MUA (Mail User Agent, such as outlook express, foxmail, etc.) that request to send mail will connect to port 25 of the email server and request to send mail. The SMTP session process usually starts with the remote identification of one's own identity, 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 is not necessarily the local domain name. At this time, the local system may have two answers: 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 allows relay, which receives and agrees to deliver a message whose destination address does not belong to the local domain name, while the second case does not receive non-local mail.

Email generally has a configuration file that determines whether or not to accept an email. The message is accepted only if the domain name of the recipient address in a RCPT TO command exists in the file, otherwise the message is rejected. If the file does not exist, all messages will be accepted. When a mail server relay all messages regardless of the recipient and who the recipient is, the mail server is called Open forwarding (open relay). When the email server does not set a forwarding limit, it is open to forward.

3. How does sendmail restrict email forwarding

Because Send Mail is afraid of becoming an advertising relay station, it only sends letters to users on the local computer after installation, so you need to modify something to get it to transfer messages for you. Modify / etc/mail/access file to add the following lines.

Localhost RELAY 192.168.1 RELAY

You can transfer the letters from the 192.168.1 domain.

This is the end of the article on "how to realize the forwarding function of Sendmail server under Linux". I hope the above content can be of some help to you, so that you can learn more knowledge. if you think the article is good, please share it for more people to see.

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