Problem sending emails from debian 8 sendmail server?

Question

PHP

Deployed sendmail on the server, the domain sends a letter from the domain, but it is not sent, here are the logs
/var/log/mail.log :

May 11 15:24:44 k24-tempdopserv sendmail[2622]: v4B9OiEt002622: Authentication-Warning: k24-tempdopserv.kz: www-data set sender to [email protected] using -f
May 11 15:24:45 k24-tempdopserv sendmail[2622]: v4B9OiEt002622: [email protected], size=948, class=0, nrcpts=1, msgid=<[email protected]>, [email protected]
May 11 15:24:45 k24-tempdopserv sm-mta[2623]: v4B9OjrU002623: from=<[email protected]>, size=1182, class=0, nrcpts=1, msgid=<[email protected]>, proto=ESMTP, daemon=MTA-v4, relay=localhost [127.0.0.1]
May 11 15:24:45 k24-tempdopserv sendmail[2622]: v4B9OiEt002622: [email protected], [email protected] (33/33), delay=00:00:01, xdelay=00:00:00, mailer=relay, pri=30948, relay=[127.0.0.1] [127.0.0.1], dsn=2.0.0, stat=Sent (v4B9OjrU002623 Message accepted for delivery)
May 11 15:24:45 k24-tempdopserv sm-mta[2625]: STARTTLS=client, relay=aspmx.l.google.com., version=TLSv1/SSLv3, verify=FAIL, cipher=ECDHE-RSA-AES128-GCM-SHA256, bits=128/128
May 11 15:24:46 k24-tempdopserv sm-mta[2625]: v4B9OjrU002623: to=<[email protected]>, delay=00:00:01, xdelay=00:00:01, mailer=esmtp, pri=121182, relay=aspmx.l.google.com. [IPv6:2a00:1450:4010:c01::1b], dsn=5.0.0, stat=Service unavailable
May 11 15:24:46 k24-tempdopserv sm-mta[2625]: v4B9OjrU002623: to=<[email protected]>, delay=00:00:01, mailer=local, pri=121182, dsn=5.1.1, stat=User unknown
May 11 15:24:46 k24-tempdopserv sm-mta[2625]: v4B9OjrU002623: to=postmaster, delay=00:00:01, mailer=local, pri=121182, dsn=5.1.1, stat=User unknown
May 11 15:24:46 k24-tempdopserv sm-mta[2625]: v4B9OjrU002623: v4B9OkrU002625: postmaster notify: User unknown
May 11 15:24:46 k24-tempdopserv sm-mta[2625]: v4B9OkrU002625: to=MAILER-DAEMON, delay=00:00:00, mailer=local, pri=0, dsn=5.1.1, stat=User unknown
May 11 15:24:46 k24-tempdopserv sm-mta[2625]: v4B9OkrU002625: to=postmaster, delay=00:00:00, mailer=local, pri=0, dsn=5.1.1, stat=User unknown
May 11 15:24:46 k24-tempdopserv sm-mta[2625]: v4B9OkrU002625: v4B9OkrV002625: return to sender: User unknown
May 11 15:24:46 k24-tempdopserv sm-mta[2625]: v4B9OkrV002625: to=MAILER-DAEMON, delay=00:00:00, mailer=local, pri=0, dsn=5.1.1, stat=User unknown
May 11 15:24:46 k24-tempdopserv sm-mta[2625]: v4B9OkrU002625: Saved message in /var/lib/sendmail/dead.letter

Answer 1

the problem of sending to "User unknown" , please tell me where to dig further?