Running Exchange2007, internal mail is fine but not recieving any mail from outside sources

We are running Exchange 2007 and all of a sudden, users cannot recieve email generated from outside the network. Email internally and email outbound to another destination works fine. As far as I can determine, senders are not getting any NDR's as a result of the email not arriving.
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Antonio VargasMicrosoft Senior Cloud ConsultantCommented:
First you need to specify your infraestructure. Is port 25 opened on your firewall allowing the mail to come in? Is the mail reaching exchange? if it worked before and it's not working now, without any changes, probably mail is not reaching exchange and you need to understand why. Your firewall must be allowing port 25 traffic in and must be tranlating the entry. You probably need to say that everything that comes from port 25 to your external ip x.x.x.x goes to port 25 on your internal ip x.x.x.x.
is it going directly to exchange or do you have a mail relay and filter system?

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