title: Sendmail: can't send mail even the stat=send
body: <p>It was working previously but one day suddenly we cannot send out any mail. Sometimes the maillog show the stat as "Data Format Error" or "send". but neither it's actually sending the mail.</p>
  
  <p>On the same day that it stops working, we had a problem that we lack disk space but we have already free out some space. Wonder if the disk space does with anything. </p>
  
  <p>We checked our firewall setting, it should be fine.</p>
  
  <p>This is part of our maillog:</p>
  
  <pre><code> Jun 16 12:31:26 exampleserver sendmail[30535]: u5G4VQFU030535: from=root, size=74, class=0, nrcpts=1, msgid=&lt;201606160431.u5G4VQFU030535@exampleserver&gt;, relay=root@localhost
  Jun 16 12:31:26 exampleserver sendmail[30536]: u5G4VQdk030536: from=&lt;root@exampleserver&gt;, size=320, class=0, nrcpts=1, msgid=&lt;201606160431.u5G4VQFU030535@exampleserver&gt;, proto=ESMTP, daemon=MTA, relay=localhost [127.0.0.1]
  Jun 16 12:31:26 exampleserver sendmail[30535]: u5G4VQFU030535: to=testing@destination.com, ctladdr=root (0/0), delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=30074, relay=[127.0.0.1] [127.0.0.1], dsn=2.0.0, stat=Sent (u5G4VQdk030536 Message accepted for delivery)
  Jun 16 12:31:26 exampleserver sendmail[30538]: STARTTLS=client, relay=testing2.destination.com., version=TLSv1/SSLv3, verify=FAIL, cipher=DHE-RSA-AES256-GCM-SHA384, bits=256/256
  Jun 16 12:31:26 exampleserver sendmail[30538]: u5G4VQdk030536: to=&lt;testing@destination.com&gt;, ctladdr=&lt;root@exampleserver&gt; (0/0), delay=00:00:00, xdelay=00:00:00, mailer=esmtp, pri=120320, relay=hku-nsp2.destination.com. [147.8.145.28], dsn=5.6.0, stat=Data format error
  Jun 16 12:31:26 exampleserver sendmail[30538]: u5G4VQdk030536: u5G4VQdk030538: DSN: Data format error
  Jun 16 12:31:26 exampleserver sendmail[30538]: u5G4VQdk030538: to=&lt;root@exampleserver&gt;, delay=00:00:00, xdelay=00:00:00, mailer=local, pri=31526, dsn=2.0.0, stat=Sent
  </code></pre>
  
  <p>Will anyone able to tell where the problem is? Thank you very much!</p>