Newer versions of the ESA AsyncOS (8.0.1-023+) have new features to help in this regards. Message Filters can now be created with the following conditions:
Recipient Count : rcpt-count How many recipients is this email going to?
Address Count : addr-count() What is the cumulative number of recipients? This filter differs from the rcpt-count filter rule in that it operates on the message body headers instead of the envelope recipients.
These can help identify potential compromised workstations or email accounts that are being used to send out a large volume of messages over a short time.
There is much more information on this in the Email User Guide:
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...