title: mutt : is not a mailbox. Could not send the message
body: <p>I have a problem to send email.<br>
  error info:<br>
  /home/debian8/Mail/sent/ is not a mailbox.<br>
  Could not send the message.   </p>
  
  <p>Here is my .muttrc configuration.</p>
  
  <pre><code>set folder="/home/debian8/Mail"
  set editor="vim"
  set sendmail="/usr/bin/msmtp"
  set use_from=yes
  set realname="xxxx"
  set from=xxxx@xxxx.com
  set envelope_from=yes
  set mbox_type=mbox
  set mbox="/home/debian8/Mail/inbox"
  set spoolfile="/home/debian8/Mail/inbox"
  set postponed="+postponed"
  set record="+/sent/"
  set check_new=yes
  </code></pre>
  
  <p>To list directory Mail.</p>
  
  <pre><code>ls Mail
  inbox  postponed  sent
  </code></pre>
  
  <p>How to fix it?</p>