Yes you are correct, by disabling the SMTP fixup protocol you are creating a SMTP security problem. I don't believe there is a way of customizing mailguard to protect SMTP and still allow ESMTP.
You can only have one or the other, i.e. fixup protocol SMTP 25 OR no fixup protocol SMTP 25. The PIX is doing its job correctly and if you are running ESMTP then I would suggest that you fine tune you ESMTP server and not the PIX! I am not a Microsoft guy so will not be able to advise on fine tuning an ESMTP server but there are lots of information on this on the web just try Google, i.e. search for PIX ESMTP problems.
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...