[exim] About adding user

Hi at all,
i've a question about adding a new user and enable him to use email
account. I've have debian linux distro. I know that to enable user to
use email account, I must add him to the system (use adduser command)
and then user can check and send email.

But suppose that I must create 1.000 email account, then according
with those write above, I would have to add 1.000 users to my system
and then permit them to use all services present into the system, e.g.
ftp, ssh, etc.

Can i enable user to use email account without add him to the system
(using adduser) with exim ??Exist some authentication mechanism that
permit it ?
Looking at exim, I know that each user have, also a system account, a
mailbox space usually in /var/mail/<user>. Then using another
mechanism i however create his mailbox space to enable email, is this
correct ??

I have read about cyrus, can I realize such mechanism using it ??
Please help me,
any kind of help is granted.
regards,
mulp