[Exim] Question about POP boxes

Hello

I was looking to add a few POP boxes to my system, so I figured that
adding a user to the system without a home directory and with no shell
would serve the purpose just fine.

After some tinkering, I discovered that the user needs a home directory,
even though mail is being delivered to /var/spool/mail/${local_part}.

Why is this? If mail is being delivered to the above directory, it
shouldn't need a home directory for the user.. or is this just some weird
configuration error on my part? :-)

I'm using Debian Unstable, 2.2.18 kernel.

Thanks,

Tim