How do I lock down my exchange server 2010

I have an exchange server 2010 and I need to 'lock down' the server.
 I am using message labs as my spam filter but spam emails are still getting
through by sending directly to the server and not following the MX record which
points to Message Labs.
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Alan HardistyCo-OwnerCommented:
You need to determine the IP Addresses that message labs use to send you mail, then restrict your receive connector to just allow those IP Addresses and this will stop mail being sent directly to you.

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If your mail server is behind firewall, create firewall rule to only accept SMTP  (port 25) traffic only from Message Labs servers' public ip address. You should can get the ip address list from contacting message labs support.
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