I have only done this for Unity Connection and a firewall, never had the need for IMAP over the internet with a UC500.
As long as you dont have email ports open to an exchange already, or if you have another outside IP to use, you can try by going to CCA > NAT and open up port 25 and 143 and point them to the CUE (10.1.10.1 default.) Never tested this, but should work in theory. On a side note, IMAP is not Unified Messaging.
I am actually going to be testing this either next week or soon after
The theory is if there is a screening router (Edge WAN router) that sits before the UC, you would port forward the IMAP port to the UC and in particular steer it to the CUE's IP address, this would mean that the EDGE router would at least need to know about that route.
However if the UC is the concentrator and there is no screening router, then in theory you could use IMAP and you could do it from an iPhone or Android device (Well this is what I am going to test it on anyway).
Right now I have my iPhone (Soon to Android I hope), conecting to the exchange server and through that it is getting the voice mails, but it would be good to see if a direct connection would work as well.
In CCA you would enable Unified Messaging, which is in the Smart Applications Draw (Damn my memory is bad this weekend) or at least in that section, this will enable IMAP for you
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