I've got 2 Ironports C350 that are relaying the email from my exchange 2003 server to the internet. Recently, users have complained that it takes up to 10min for email to reach their destination. After investigation, emails seems to stay for a long time in the exchange SMTP queue as queued messages. Enabling the transport log shows the following error occuring once in awhile :
Event Type: Warning
Event Source: MSExchangeTransport
Event Category: Connection Manager
Event ID: 4006
Time: 2:27:35 PM
Message delivery to the host '192.168.1.40' failed while delivering to the remote domain '[192.168.1.40];[192.168.1.41]' for the following reason: The connection was dropped by the remote host. (192.168.1.40-41 being my C350s running 6.5.1-005)
I'm assuming the problem is with the ironports because if I remove the smarthost config in exchange and deliver the email directly from the exchange server using DNS, deliveries are back to near instantaneous.
I've tried to change the Max Messages per Connection from the default 10 to 100 and changed the Max. Concurrent Connections from a Single IP from the default 2 to 5. I haven't noticed much improvement from those changes. If I wait long enough all emails eventually gets delivered.
Where would you start to troubleshoot this issue?