I am guessing you have a route on your ASA5510 that routes 10.0.0.0/8 to the inside. What you will need to do is add a static route for 10.50.0.0/16 and point it toward your ISP. The error message you are seeing is saying that (as far as it knows) traffic is going from 'inside' to 'inside' which really isn't possible without some out of the ordinary configuration on the ASA.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...