If there is no NAT configuration then the ASA must be routing your subnet block behind the ASA, and the upstream must be NATing that for you. Regretfully, it seems that you wont be able to simply transfer the IP address from the ASA to the MX and have it work, since that IP address has no access to the Internet. You will need your service provider to provide you with a subnet that can be applied to the outside of the MX that they will NAT for you, or a directl public IP address. Sorry I couldn't resolve this one for you.
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