Exchange 2000 cannot send/receive mail from Outside

Cannot send or receive mail from external sources. Can send and receive within our LAN. Exchange 2000 running on Server 2000.
Everything has been good since this server was deployed last summer. We had a week off at our school. (Spring Break). The day after we came back the mail stopped working. We can only send/receive within our LAN. Nothing is comming in or going out from the outside. I have rebooted the server and restarted the various related services. I am not very experienced with Exchange. Please give me a hand I am the only one here to support 150 computers and 5 servers. I will reboot the other 3 servers in the meantime. I also ran a DNS Report. 1 says no reverse DNS, the other says I may be on a spam list.
gpfrankAsked:
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MCPJoeCommented:
If you are getting absolutely no mail in our out, the spam blacklist is the least of your worries...

I suggest taking a look at your network config.  Changes are you have some type of router or firewall that forwards a port (25) to your Exchange server.  It could be that this firewall or router is malfunctioning.  Its also possible depending on your situation that your IP may have changed.  This would affect your DNS records for your domain and your MX records may be pointing to outdated IP addresses that no longer work.  
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jhoop2002Commented:
did any updates get applied?  Also, double check that everything (servers, firewall, routers) have static IP address's (also make sure they are the right address).
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gpfrankAuthor Commented:
Sorry it took me so long to get back in here. I am the only one supporting 135 PCs, 5 servers, and a large quanity of other devices. No one hit the nail on the head. However, the sugested answers kicked my feeble brain into gear and I found a way to fix it. Thank you for your valuable input.
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jhoop2002Commented:
what did you end up needing to do?
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gpfrankAuthor Commented:
I forgot to post the fix. I have an aging ISA 2000 server that was holding up external traffic. A reboot fixed it. I will be replacing that piece of iron very soon. Thanks again.  Glenn
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