sendmail + dovecot + outlook problem

I installed Debian Sarge on my network as proxy server now this
machine is working as a proxy very well. Our client need local mail
server I would like to plan uses this proxy server as a local mail
server. In our network all client machine are windows XP so we would
like to use local mail through outlook mail client.
I think sendmail and dovecot can solve this problem so I just
installed sendmail and dovecot and try to send mails through outlook
but it's not working. I think I have to modify sendmail and dovecot
configuration files. How I can fix this problem. Please help me.