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

[Niek Dekker]

Hi,

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

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

Many thanks i.a.
Niek