We run Exchange 2003 SP2. Our servers send/receive mail to/from virtually all domains fine. We have one customer, however that we can't seem to receive any largeemails from. They also run Exchange 2003, SP2. Here is what the conversation looks like between our servers: From their server: HELO mail.client.com MAIL FROM:<client@client.com> RCPT TO:user@ourserver.com DATA IDI3OCAwIDAgMCAwI........ From our server: 220 mail.ourserver.com Microsoft ESMTP MAIL Service, Version: 5.0.2195.6713 ready at Fri, 13 Jun 2008 08:42:17 -0600 250mail.ourserver.com Hello [ip_address]250 2.1.0 client@client.com....Sender OK250 2.1.5user@ourserver.com 354 Start mail input; end with <CRLF>.<CRLF> After this negotiation, their server will send a 1,380 byte chunk of data. Then it will wait around 1 minute and send another 1,380 byte chunk. Eventually, after sending (usually) somewhere between 15k and 30k of data, thesession will drop without ever finishing. The net result is, small text emails will come through OK. But larger emails or emails with attachments will never make it through as the session drops before it ever finishes. Based on the funky behavior of the delay between every chunk of data, I suspect the problem is on their end, but any input that might help me (or help me help them) would be greatly appreciated. Thanks in advance.

Clarify: user will not receive large mails or the mails with attachment from a particular customer company. Collect unclearly info to isolate issue: 1. Are there any similar symptoms when other customers send larger mails to your users? 2. Which anti-virus product is installed on your exchange server? Note: if all other customers can send larger mail to your org, only except the particular customer, we can assume theres no issue on your side Troubleshooting on your side: Basic: You can request a free mail account from yahoo, and then send a larger mail with attachment to your mailbox for testing Advanced: 1. Try to set 100MB Full Duplex in the exchange servers NIC configuration 2. Turn off checksum offloading (if have, itll checkout network package to release the burden of CPU, but it can also cause receiving issue for larger mail. Please test in un-product time) Notes: This will disconnect you from the internet for a brief moment. You should automatically reconnect, if not restart your machine Right-click your active network connection and select Properties ->click configure button-> Advanced tab->choose CHECKSUM OFFLOAD and change it to Disable For the issue on customers side, youd better let customer to post a thread for analysis on the forum Like: Is there any NDR being returned to customers users if the mail werent sent? Is the issue only happening on the mails with your organization? Check the unsent mails mail flow by SMTP logging http://www.computerperformance.co.uk/exchange2003/exchange2003_logs_SMTP.htm#Creating%20SMTP%20logs

James, Thanks for the response. In isolating the issue: 1) I have not seen this behavior with any other sender. We have tested sending large attachments from GMail, Yahoo, and two other Exchange servers. We have had no complaints from any other customers about sending attachments. We do have other customers who send lots of large attachments, which we get with no problem. In short, I believethe problemis isolated to this one sender. 2) We are running McAfee Groupshield. As for the NIC settings: 1) NIC is already running at 100mb/s full duplex. 2) There is no adjustable setting on this NIC for checksum offloading. I guess where that leaves us that it is a problem on the sender's side and they will have to solve it. Thanks again.

Good to know that's ok on your environment. You can ask your customer to post a thread at this forum. It would help him to narrow down the root cause Exchange: http://forums.microsoft.com/TechNet/default.aspx?ForumGroupID=235&SiteID=17

Wondering if there was ever a resoultion to this. We are experiencing the same issue with a client. We can receive emails with no or small attachments, but larger (3 MB for example) fail.

I know this is a long shot but can the customer send to other organizations large attachments?Sometimes when people use DSL for their connections they need to have their MTU lowered to 1492 on their external router for SMTP to send correctly.Just a hunch.Good luck