# Postfix smtp-auth
  unix_listener /var/spool/postfix/private/auth {   
     mode = 0666
    user = postfix
    group = postfix
  }

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The mode is a permissions mask that allows read and write, but NOT execute, for the owner, group and everyone else.
The user and group specifes who gets to listen to authentication commands. It is crucial you specify these fields, although in alternate configurations this might not be needed.