smtp security for hmail server

we are hosting multiple domain email server inside our company ,we found that our IP address has been blacklist .May i know how to configure only the domain user able to send msg .Because l realize actually i just telnet smtp port then can use the server send msg without knowing the username and password.
YaYangTeahAsked:
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KimputerCommented:
Ouch, any internet facing SMTP server should have some kind of security, and you should have done this the moment you decided you wanted your SMTP server to relay.
What you are searching for is in the Settings > Advanced > IP Ranges. Find the IP range that's causing this (by default it's called Internet) and enable the several Require SMTP Authentication checkboxes (leave external to local blank, otherwise incoming mail will stop).
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