We use the Microsoft Exchange Hosted Filtering Service for all of our inbound
and outbound e-mail. Lately, we have had a large spike in e-mails with large
attachments that are not coming through in a timely fashion. It seemed to
get worse after we applied SP2 for server 2003, but there were deferrals
before that point, so it is hard to say if that is conclusive. We use CA
etrust AV connector for Exchange V7.0, which I have disabled (did not seem to
make much difference), then uninstalled (as per a forum post which said that
as AV sinks are tightly integrated with Exchange, disabling alone might not
resolve the problem). There seemed to be some improvement after uninstalling
the AV connector, but that is hard to say. I am basing it off the number of
messages in deferral at any given time.

If I run a deferral report in the EHS admin centre, these are the types of
messages I am seeing:

2/27/2008 11:00:00 AM conversation with [x.x.x.x] timed out
while sending message body

2/27/2008 11:07:00 AM conversation with [x.x.x.x] timed out
while sending end of data -- message may be sent more than once.

The problem only seems to occur with messages with large attachments (above
1.5 MB). EHS tech support has not been much help, as they say the problem is
on our end.

Also, if I look at "current sessions" under the SMTP virtual server, there
are usually several (up to ten at a time) sessions in there from the EHS
servers (SMTP is closed to all other IPs) with connected times sometimes over
600 seconds (I think EHS times out after approx. 10 minutes).

Researching this problem has not turned up much. The "path MTU discovery"
issue comes up frequently, but that would be a problem for outbound e-mail,
not inbound.

I am not enough of a network guru to analyze this with packet traces. Any
suggestions?

We are using Exchange 2003 SP2, with an Astaro firewall appliance. Our
internet connection is a fixed wireless, 1.5 MB symettrical.

Archived from group: microsoft>public>exchange>connectivity