New to list- how to kill relay

[visionary@xxxxxxxxxxxxx (Alan Rafael Bleiweiss)]

Happy holidays to all!

I'm new to the list though I've been using PO as my mail server for a 
couple years.  I've got one server, and about 50 email accounts 
spread across several domains I host... most of these are non-profit 
companies, but several are ecommerce.


I'm frustrated cause I'm on a couple "spam relay" lists because of my 
configuration where I approve local domains for relay - clients are 
all over the country and on many different ISP networks, and so some 
of the email that goes through as legitimate site purchase 
confirmations and correspondence get rejected as a result.  NOT good 
for business!!!

Anyhow - I'm so totally confused about how to lock down the server to 
prevent relaying - I did read the FAQ on this but if I uncheck "and 
allow local domains - but just list my network IP, doesn't that mean 
that only people who are inside my LAN can get and send EMail?

If that's the case, can I create a web based interface inside my LAN 
that clients all over the country can use to send and receive email - 
requiring them to log in using a network password I'd need to create 
for each of them?

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