You have been installing sendmail according to the previous installation, but you can’t send email this time. Check the email log and report an error

Jun 18 11:34:23 iZ255lfo12hZ sendmail[4859]: t5I3YJd0004857: to=< [email protected]> , delay=00:00:03, xdelay=00:00:03, mailer=esmtp, pri=120430, relay=mx3.qq.com. [183.57.48.35], dsn=5.0.0, stat=Service unavailable
Jun 18 11:34:23 iZ255lfo12hZ sendmail[4859]: t5I3YJd0004857: to=< [email protected]> , delay=00:00:03, mailer=local, pri=120430, dsn=5.1.1, stat=User unknown
Jun 18 11:34:23 iZ255lfo12hZ sendmail[4859]: t5I3YJd0004857: t5I3YNd0004859: postmaster notify: User unknown
Jun 18 11:34:23 iZ255lfo12hZ sendmail[4859]: t5I3YNd0004859: to=root, delay=00:00:00, xdelay=00:00:00, mailer=local, pri=31639, dsn=2.0.0, stat=Sent

There is no service, stat = service unavailable, failed to send. My email address also tells stat = user unknown that there is no such user. I have tried all kinds of methods to find information on the Internet, but there is no result. But to 163 mailbox can send mail, and finally found a solution

Solution: resolve the sender’s domain name address to the mail server