We've been getting NDR's from users sending attachments over 1mb.
We run a SBS 2003 server with exchange built in and all the current service pac's (recently updated to server 2003 sp2). The use our ISP to fwd our mail to and a pop connector, so the exchange isn't "live" and we have no MX records. The SBS box runs 2 nic's with the WAN going through a snapgear firewall and a netgear adsl modem.
The messages get stuck in the smallbusiness smtp connector queue with an error "The connection was dropped by the remote host."
the NDR's we get are
This is an automatically generated Delivery Status Notification.
THIS IS A WARNING MESSAGE ONLY.
YOU DO NOT NEED TO RESEND YOUR MESSAGE.
Delivery to the following recipients has been delayed.
before we get this one
The following recipient(s) could not be reached:
on 24/01/2008 10:11 AM
Could not deliver the message in the time limit specified. Please retry or contact your administrator.
All users can send mail fine as long as it dosn't contain an attachment over about 1 meg (not sure of the exact size but 1 meg on average)
I've checked all the timeout settings and size limit settings on the server and they appear to be normal. furthermore No-one has changed anything manually anyway. I've spoken to the ISP and they say the problem is at my end and there's nothing more they can do. I've checked the MTU settings and looked for black hole routers in the chain somewhere to no avail.
I've tried Turning off EDNS-0 extended UDP packets re this link http://www.nthelp.com/w2k3/turning_off_edns.htm
there are a few good articled in EE but most don't apply to my scenario as I'm not running a live exchange.
I'm running out of ideas and expertise
can anyone help?