We noticed a while ago that emails were suddenly disappearing without trace, no NDR being genereated. The email just seems to disappear.
There are multiple ways to resolve this issue but I'll explain my method which will hopefully point most people in the right direction.
Firstly you need to find out exactly what the issue is, in my case we tracked the message in ESM and found the message was received by the server but then never delivered with the message; "SMTP: Advanced Queue Failed to Deliver Message in ESM"
You should enable message tracking, by right clicking the mail server in ESM and checking the tracking box on the general properties page.
Review the log file to find the exact event id from the exchange server.
An example of my case is as follows:
10:33:38 GMT SERVER2K3 firstname.lastname@example.org 1019
10:33:38 GMT SERVER2K3user1@domain2.co.uk 1025
10:33:44 GMT SERVER2K3 email@example.com 1026
You should use the following link to reference the event ID's
Message tracking event IDs in Exchange Server 2003
Event 1026 is most likely caused by the 3rd party Anti Spam software
running on exchange server.
Either the message was detected as spam or there may have been some corruption in the message. McAfee GroupShield installed on the Exchange server can also cause this.
We actually found that by disabling the scanning function in Group shield this resolved the error.
I have also however noticed in the past that GFI can cause this issue to, you should look in the Program files directory under GFI for any emails, you will most likely find them there.
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