Following is message with show conn in the pix525,ver 7.0. x.x.x.x is email server ip address. I don't think have 4g email send out in that time. How to understand this message? that source ip address is changed as time.
"TCP out 62.37.236.x:25 in x.x.x.x:28626 idle 0:07:05 bytes 4294967280 flags UO"
It's possible your mail server has sent 4GB+ of email over time. If you haven't modified the 'conn timeout' then the TCP session will terminate in 1 hour (inactivity) including the idle-time, byte count, and flags. The idle-time is probably throwing you off here.
Your SMTP server banner will be exposed and you won't be able to inspect RFC eSMTP commands. Most customers who run IIS or Exchange servers keep the inspection off anyways since in most cases causes problems with inbound and outbound email.
It seems as pointed out earlier that a inside host x.x.x.x did send a mail 4GB or more, however the connection was not a normal finish, it was torn down becuse the mail server on the outside RESET the connection. Notice the flags in the syslog-
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...