Other MTAs, like Qmail or Courier, use a more modern INBOX format called “maildir” (Exim4 can also use the maildir format if configured appropriately). The differences between “mbox” and “maildir” are two-fold:

1) the huge monolithic INBOX file is now split into a lot of different files, and

2) the INBOX is now usually stored in the user's home directory. So, for example, the INBOX for user john might be located in the directory

/home/john/Maildir

So in this case the MTA delivers mail straight into the user's home directory!!!