etc/dovecot/sieve/global.d/extension.sieve

   1 require
   2  [ "envelope"
   3  , "fileinto"
   4  , "mailbox"
   5  , "subaddress"
   6  , "variables"
   7  ];
   8
   9 if envelope :matches :detail "TO" "*" {
  10         set "extension" "${1}";
  11  }
  12 if not string :is "${extension}" "" {
  13         fileinto :create "Extensions.${extension}";
  14         stop;
  15  }