I have used sendmail for the sending mail. Suddenly mail is stop sending for all my linodes. Here is the last five line of my mail.log file.
Jul 27 17:48:01 ubuntu sm-mta[23238]: v6RHm132023236: v6RHm132023238: DSN: Service unavailable
Jul 27 17:48:01 ubuntu sm-mta[23238]: v6RHm132023238: to=<root@ubuntu.members.linode.com>, delay=00:00:00, xdelay=00:00:00, mailer=local, pri=30000, dsn=2.0.0, stat=Sent
Jul 27 17:50:01 ubuntu sendmail[23293]: v6RHo1g0023293: from=root, size=724, class=0, nrcpts=1, msgid=<201707271750.v6RHo1g0023293@ubuntu.members.linode.com>, bodytype=8BITMIME, relay=root@localhost
Jul 27 17:50:01 ubuntu sm-mta[23294]: v6RHo1aX023294: from=<root@ubuntu.members.linode.com>, size=1023, class=0, nrcpts=1, msgid=<201707271750.v6RHo1g0023293@ubuntu.members.linode.com>, bodytype=7BIT, proto=ESMTP, daemon=MTA-v4, relay=localhost [127.0.0.1]
Jul 27 17:50:01 ubuntu sendmail[23293]: v6RHo1g0023293: to=sagar.prajapati@multidots.com, ctladdr=root (0/0), delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=30724, relay=[127.0.0.1] [127.0.0.1], dsn=2.0.0, stat=Sent (v6RHo1aX023294 Message accepted for delivery)
Jul 27 17:50:01 ubuntu sm-mta[23296]: STARTTLS=client, relay=aspmx.l.google.com., version=TLSv1.2, verify=FAIL, cipher=ECDHE-RSA-AES128-GCM-SHA256, bits=128/128
Jul 27 17:50:02 ubuntu sm-mta[23296]: v6RHo1aX023294: to=<sagar.prajapati@multidots.com>, ctladdr=<root@ubuntu.members.linode.com> (0/0), delay=00:00:01, xdelay=00:00:01, mailer=esmtp, pri=121023, relay=aspmx.l.google.com. [IPv6:2607:f8b0:400e:c05:0:0:0:1a], dsn=5.0.0, stat=Service unavailable
Jul 27 17:50:02 ubuntu sm-mta[23296]: v6RHo1aX023294: v6RHo2aX023296: DSN: Service unavailable
Jul 27 17:50:02 ubuntu sm-mta[23296]: v6RHo2aX023296: to=<root@ubuntu.members.linode.com>, delay=00:00:00, xdelay=00:00:00, mailer=local, pri=30000, dsn=2.0.0, stat=Sent
