Postfix creates email bounces with permissions set to 600, but CiviMAIL doesn't like that!
Avoid using bin/emailprocessor.php, and instead configure your mail processor using the instructions under Configuring inbound email processing on CiviCRM's Email system configuration page.


You will create a mailbox accessible via network POP3 or IMAP protocols, not maildir.
You will configure a Bounce Processing mailbox connection under CiviCRM's Mail Accounts (Administer > CiviMail > Mail Accounts, civicrm/admin/mailSettings) to access this mailbox.
You will configure CiviCRM's Scheduled Jobs (Administer > System Settings > Scheduled Jobs, civicrm/admin/job) to run the Fetch Bounces Job periodically.
You will confirm that your system cron configuration is correctly triggering this job to run.


With a POP3 or IMAP server handling the interaction between CiviCRM and your server's MDA, permissions issues should never arise. The files will be read by the mail system (eg Courier) as an appropriate system user, and will be communicated over the (local) network connection to CiviCRM's bounce fetcher task. Much cleaner.



CiviMail Processor in the CiviCRM wiki has a pink warning box at the top (which is hidden in mobile view): 


  Not Up to Date
  
  This page isn't up to date anymore as it references a deprecated script: /bin/CiviMailProcessor.php. The new method of handling incoming mail is described on this book page.