For the past couple months some of my users have been complaining of a somewhat odd e-mail issue. The problem manifests as multiple duplicate e-mails from a single, known sender.
The characteristics of the duplicate messages are as follows:
Same message ID
Identical header information, aside from timestamp on the last hop
Sometimes with attachments, sometimes without
Usually sent to a number of people, generally >5
Messages arrive in ~1 hour intervals
A simple diagram of our network looks as follows:
1) Trusted: Users
2) Trusted: Exchange 2003
3) DMZ: GFI MailSecurity / MailEssentials in SMTP Gateway mode
4) Cisco PIX-525
The timestamp discrepancy occurs between numbers 3 and 4.
If you would like to punish your eyes, you can view the Exchange Message Tracking information for the MessageID in question at this link: http://www.clarionlogic.com/tracking.xml
There doesn't seem to be any real pattern as to when this happens or to whom. We use Symantec AntiVirus on the Exchange server, with all the necessary files/processes excluded from scanning. There doesn't seem to be anything 'stuck' in either the Exchange or GFI server mail queues.
Any help will be greatly appreciated. If you need further information, please ask and I will do my best to provide it.