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Get to the bottom of this frustrating bug in Exchange

This article examines a frustrating bug in Exchange with a few suggestions for workarounds.

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Have you encountered this bug before? When you send mail to an Exchange 2003 server, it returns some messages as nondeliverable, with the following error message from the recipient's Exchange server:

421 4.1.0 Dropping connection due to an error on this server

This bug is more frustrating than most. There are no articles in the Microsoft Knowledge Base that address this issue. And even with all applicable diagnostics logging turned up to maximum, the system logs nothing that offers any information other than the original nondelivery report (NDR).

In addition, this is a limited problem; Exchange doesn't reject most messages. Because the problem occurs on a limited basis, Exchange administrators on both sides of the problem often feel that it isn't their problem, particularly when no one else is complaining.

But if you're experiencing this problem, it can definitely be frustrating. Here's some information that may help.

The bug appears to occur only under a specific set of circumstances. The recipient's Exchange 2003 Server must have Sender Filtering enabled. But even if you've enabled Sender Filtering, the server only rejects messages when they don't contain an empty line after the SMTP DATA command during the message's transmission. (These messages are technically malformed, but most mail servers will accept them.)

To fix this problem, start at the source. The client that's sending the message isn't forming the message properly, so approach the issue from that angle.

In some cases, however, there may be no effective way to "fix" the client, so the only solution is a workaround. Turning off Sender Filtering on the recipient's side will alleviate the problem, but some IT departments have turned on this feature for good reasons.

A real solution can only come from Microsoft in the form of a service pack or hot fix. In the meantime, we hope that understanding the bug and its triggers may help you find a workaround.

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