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DO you have any anonymous Receive connector? You should be able to add this external ip address as the same way you allow internal Server relays
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Make sure that you follow the external relay from the link:<br />
<a href="http://technet.microsoft.com/en-us/library/bb232021%28EXCHG.80%29.aspx" rel="ugc">http://technet.microsoft.com/en-us/library/bb232021%28EXCHG.80%29.aspx</a><br />

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Thanks for the response rr, would this prohibit other external IP addresses from delivering to Exchange?
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You are receiving your emails only from the cloud provider, right?<br />
Who holds your MX record?
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I recently started using in the cloud spam filtering which scrubs my emails and then delivers the email to my Exchange 2007 server, but I do not have a firewall advanced enough to allow only specific IP addresses sending SMTP traffic to my Exchange 2007 server. Is there a way in Exchange 2007 to create a list of IP addresses that Exchange allows in? Also, would this even work for in the cloud spam filtering delivery?
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I recently started using in the cloud spam filtering which scrubs my emails and then delivers the email to my Exchange 2007 server, but I do not have a firewall advanced enough to allow only specific IP addresses sending SMTP traffic to my Exchange 2007 server. Is there a way in Exchange 2007 to create a list of IP addresses that Exchange allows in? Also, would this even work for in the cloud spam filtering delivery?

Accepted receive list on Exchange?

Within Internet message handling services (MHS), a message transfer agent or mail transfer agent (MTA) or mail relay is software that transfers electronic mail messages from one computer to another using a client–server application architecture. A MTA implements both the client (sending) and server (receiving) portions of the Simple Mail Transfer Protocol (SMTP). The terms mail server, mail exchanger, and MX host may also refer to a computer performing the MTA function. The Domain Name System (DNS) associates a mail server to a domain with mail exchanger (MX) resource records containing the domain name of a host providing MTA services.

Email Servers

A server is a computer program or a machine that waits for requests from other machines or software (clients) and responds to them. This architecture is called the client–server model. The clients may run on the same computer or may connect to the server over a network. The purpose of a server is to share data or hardware and software resources among clients. Typical computing servers are database servers, file servers, mail servers, print servers, web servers, game servers, and application servers.

Server Software

Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.