Can one user have more than one system mailboxes?

[a at zeos.net]

Can a user have more than one system mailbox?

E. g., some ISP provides the next service: a client can make any number of
mailboxes for himself using web interface.
Almost all ISP are using UNIX.
So, how they do this?
Does that web interface create a new system user every time
I create a new mailbox?

I have sendmail 8.13.6 on FreeBSD 6.1-RELEASE.

Elisej Babenko