As Adam said, config looks good. Your smtp server 10.20.20.55 is on a different subnet than your inside interface.
Your pix has a route to 10.20.0.0 network. Does the smtp server know how to route back ie do you have a default route that sends traffic to the pix as the source IP addresses will be public addresses from the internet.
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...