How does useradd determine the default mailbox/maildir location for new users

[Lowell Gilbert]

Niek Dekker <niekdekker at gmail.com> writes:

> I am running Exim 4.62 MTA on FBSD 6.0_release.
> When I create a new user using useradd, a mailbox file for the new
> user is created in /var/mail.

I assume you mean adduser(8).

> As I am using Maildir with maildirs in /var/mail, I do not want that.

Hmm, yes.  

> My question is, where is this behavior of useradd configured?

It isn't.  It appears to be hardwired into pw(8), which is the basic
tool around which adduser(8) is built.

> I cannot find this in the handbook. There is no /etc/useradd.conf
> neither an /etc/pw.conf on my system and also the contents of
> /usr/share/skel do not seem to make a difference.

And none of those are documented to do anything of the sort.
I think you should just write your own wrapper.  You could write it as
a wrapper for adduser, or you could make your own version of adduser
(which is just a shell script).