Quoting Nick Boyce (nick@glimmer.demon.co.uk):
> Sorry for this presumably dumb question:  I've been reading the
> various debates there have been on this list about the pros & cons of
> the two mailbox formats (mbox, maildir), and am happy enough to make
> my own decision on that for now.
>
> But ... how do I know which mailbox format my system is set up to use
> by default ?
>
> This is a new NetBSD box, and basically nobody has read any of their
> mail by any method just yet.

sendmail typically uses mail.local (or /bin/mail) which uses "version 7"
mailboxes (known as "mbox" sloppily).

And if you have thousands of users or thousands of messages,
maildir will give you an advantage.  If you have to ask, it may
be that you don't need it.