Exchange 5.5 and access from outside?

ok here the Q

i current have a simple mail server running on a p100 w/64MB ram
and im moving to an Exchange server
pIII 500x2 -4GB ram 40GB HD

ive set up exchange and it works frm inside the network and outside so the MX and A records in theDNS are correct.

what i need is:
i have clients that use are email system from other email domains.
i now how to add the domains
but how do they connect to get there email with out using OWA or adding them to the doamin.
i know exchange can be used like a normal POP3/smtp or imap server but how?

i cant use anonymus accounts ot NT domain accounts?

thanx in advance
Nexus