This has been a very perplexing problem and I hope someone can shed some light on this problem. I have an Exchange 2010 box. E-mails with attachments from mulitple domains but not all are not coming through. At least two domains can send me E-mails with attachments seemingly with no problems. For the domains that I'm having prolems with if there is not attachment the E-mail comes through with no problems. In addition to the 421 4.4.1 error I also get 451 4.7.0.
When an E-mail from one of these problem domains is being sent with an attachment, I can see the connection being made to our server but the connection will eventually time out. I have reviewed the SMTPProtocol log and I can see all the normal SMTP communications. It gets down to transmitting data and it times out after what I assume is the connection timeout expiration.
I increased the connection timeout on my receive connector from the default of 10mins up to 40mins and that did nothing. I have disabled Chunking and BinaryMime all to no avail. I have lowered the MTU rate on my router, the firewall and the server and still nothing.
I did a packet capture on my firewall and in one example when the E-mail comes in with an attachment i see the 3-way handshake, I then see the data being pushed. After about 12 minutes the far side connection issues a reset and the connection is terminated. In my test the attachment is a reasonably sized attachment. The attachment size limitation in Exchange is not the issue. We can send attachment to these problem domain without any problems, the reverse however is a problem. I can seemingly telnet to port 25 on my Exchange server and send test E-mails with attachments all day. I also pushed out to my perimeter router and telneted to 25 my Exchange server (Path = Router>Fireware>Exchange) and I can send E-mails seemingly all day with attachments and they come through fine.
I also diabled ESMTP on the firewall and that had no effect. When I look in the Protocol log everything looks normal, there are no ******** that would indicate a problem. My Exchange server is receiving E-mail directly, which I know is not ideal, but never the less is the present setup. I'm in a control environment so I'm not so worried about the present setup. I'll be putting a Edge server in place as soon as possible.
Can anyone shed any light on this? I have come up empty. I have Googled the issue extensively and found plenty of hits on this issue but none of the remedies have resolved the problem. I thought we might be on to something with the MTU setting but that seems to not be the issue. I lowered my MTU to 900. I did a ping with load from a completely separate circuit and the do not fragment switch and it failed until I got to 960 bytes. Below 960 I got responses but with 40% packet loss.
After doing the ping test remotely I moved to local environment where I'm having the problem and I did a ping with payload from the perimeter router to my Exchange 2010 server, again pings above 960 failed and below I would see packet loss of 40%. The suggested to me that maybe something is wrong with the router or cabling. But to counter that why can i receive E-mails with attachment from some people. If this is the problem I would expect everyone to have the problem and my telnet test from the router should fail.
What say anyone about this problem? Nothing seemingly makes sense.