разобрался, сама BSD клеит общее положение с наличием всех начальных слешей, и их три в итоге, а POSTFIX через virtual.

Вот эта часть мануала нам говорит об этом
MAILBOX LOCATION
       The  mailbox  location  is controlled by the virtual_mail-
       box_base and virtual_mailbox_maps configuration parameters
       (see below).  The virtual_mailbox_maps table is indexed by
       the recipient address  as  described  under  TABLE  SEARCH
       ORDER below.

       The mailbox pathname is constructed as follows:

         $virtual_mailbox_base/$virtual_mailbox_maps(recipient)

       where recipient is the full recipient address.

UNIX MAILBOX FORMAT
       When  the  mailbox location does not end in /, the message
       is delivered in UNIX mailbox format.   This format  stores
       multiple messages in one textfile.

       The  virtual(8)  delivery  agent  prepends  a "From sender
       time_stamp" envelope header to each  message,  prepends  a
       Delivered-To:  message  header with the envelope recipient
       address, prepends an X-Original-To: header with the recip-
       ient  address as given to Postfix, prepends a Return-Path:
       message header with the envelope sender address,  prepends
       a > character to lines beginning with "From ", and appends
       an empty line.

       The mailbox is locked for exclusive access while  delivery
       is in progress. In case of problems, an attempt is made to
       truncate the mailbox to its original length.

QMAIL MAILDIR FORMAT
       When the mailbox location ends in /, the message is deliv-
       ered  in qmail maildir format. This format stores one mes-
       sage per file.

       The virtual(8) delivery  agent  prepends  a  Delivered-To:
       message  header with the final envelope recipient address,
       prepends  an  X-Original-To:  header  with  the  recipient
       address  as  given to Postfix, and prepends a Return-Path:
       message header with the envelope sender address.

       By definition, maildir format does  not  require  applica-
       tion-level file locking during mail delivery or retrieval.