Getting the following error in the error log when sending the email 'DSN: User unknown'

Issue

• Getting the User Unknown error when sending an email

• The following is the error log from /var/log/maillog

Dec  7 23:00:44 prod-db sendmail[14933]: oB870i23014933: from=<root@server.example.com>, size=316, class=0, nrcpts=1, msgid=<201012080700.oB870iKU014932@server.example.com>, proto=ESMTP, daemon=MTA, relay=localhost.localdomain [127.0.0.1]
Dec  7 23:00:44 prod-db sendmail[14932]: oB870iKU014932: to=user@example.com, ctladdr=root (0/0), delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=30038, relay=[127.0.0.1] [127.0.0.1], dsn=2.0.0, stat=Sent (oB870i23014933 Message accepted for delivery)
Dec  7 23:00:44 prod-db sendmail[14935]: oB870i23014933: to=<user@example.com>, ctladdr=<root@server.example.com> (0/0), delay=00:00:00, xdelay=00:00:00, mailer=smtp, pri=120316, relay=relay.example.com. [10.10.11.172], dsn=5.1.1, stat=User unknown
Dec  7 23:00:44 prod-db sendmail[14935]: oB870i23014933: oB870i23014935: DSN: User unknown
Dec  7 23:00:44 prod-db sendmail[14935]: oB870i23014935: to=<root@server.example.com>, delay=00:00:00, xdelay=00:00:00, mailer=local, pri=31561, dsn=2.0.0, stat=Sent