title: Error: channel gmail-sent: slave sent cannot be opened
body: <p>I'm able to sync my Gmail inbox, but the sent folder does not work.</p>
  
  <p>This is my <strong>.mbsyncrc</strong></p>
  
  <pre><code>IMAPStore martinstabenfeldt-remote
      Account martinstabenfeldt
  
  MaildirStore martinstabenfeldt-local
      Path ~/.mail/martinstabenfeldt/           
      INBOX ~/.mail/martinstabenfeldt/INBOX
  
  Channel martinstabenfeldt
      Master :martinstabenfeldt-remote:
      Slave :martinstabenfeldt-local:
      Patterns *
      # Automatically create missing mailboxes, both locally and on the server
      Create Both
      # Save the synchronization state files in the relevant directory
      SyncState *
  
  Channel martinstabenfeldt-inbox
      Master ":martinstabenfeldt-remote:INBOX"
      Slave  ":martinstabenfeldt-local:INBOX"
  
  Channel martinstabenfeldt-sent
      Master ":martinstabenfeldt-remote:[Gmail]/Sent Mail"
      Slave  ":martinstabenfeldt-local:sent"
  </code></pre>
  
  <p>This is the error I get:</p>
  
  <pre><code>$ mbsync --verbose  martinstabenfeldt-sent
  Logging in...
  Opening master box [Gmail]/Sent Mail...
  Opening slave box sent...
  Error: channel martinstabenfeldt-sent: slave sent cannot be opened.
  C: 1/1  B: 1/1  M: +0/0 *0/0 #0/0  S: +0/0 *0/0 #0/0
  </code></pre>
  
  <p>Any idea why this fails?
  I've created the <strong>~/.mail/martinstabenfeldt/sent</strong> folder. And this folder is empty.</p>