The only work around is to not have the Router "inspect" smtp traffic.
CBAC works like the "mailguard" on the PIX and only allows the 7 required SMTP commands through. As you have found out first hand, ESMTP does not work in conjunction with CBAC. As of right now, there is no fix for this.
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...