Explanation of Sendmail mail server forwarding function

1. Why cannot I configure the email server as open relay?
If the system administrator sets his email server to open relay, some spam senders will use your email server as a relay station for forwarding self-forwarded emails, this will enable the recipient of spam to point at you, which may lead to retaliatory email bombs. Spam can also consume a lot of your resources and occupy your bandwidth. Even worse, your name may be blacklisted and become a target shared by other Email recipients. Your email will be rejected by these recipients.
2. What is mail relay)
After an email server is configured, the server will have one or more domain names. At this time, the email server will listen to port 25 and wait for the Remote Sending of mail requests. Other mail servers on the network or MUA (Mail User Agent, such as outlook express and foxmail) sending emails will connect to port 25 of the email server to send emails, the SMTP session process generally starts from remotely identifying your identity. The process is 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's user1@elsewhere.net is not necessarily a 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. It receives and agrees to pass an email with a destination address that does not belong to the local domain name. In the second case, it does not receive non-local emails.
Generally, email has a configuration file, which determines whether to accept an email. This email is accepted only when the Domain Name of the recipient address in the rcpt to command exists in this file. Otherwise, the email is rejected. If the file does not exist, all emails will be accepted. When a mail server forwards all emails (relay) regardless of the recipient and recipient, the mail server is called open relay. When the email server does not set forwarding restrictions, it is open for forwarding.
3. How does sendmail limit Email Forwarding?
Because sending Mail is intended to become an advertisement-Based Email Forwarding site, after installation, it only sends emails to users on the local machine. To transfer emails to you, you need to modify some information. Modify the/etc/mail/access File and add the following lines.

localhostRELAY 192.168.1RELAY

You can transfer the email in the 192.168.1 domain.