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We now have a Message Re-Delivery Workaround policy option that attempts to resolve this issue by stripping the original Message-ID header field during the re-delivery process. The feature will be automatically enabled for most Exchange servers (but can be manually disabled).

Microsoft 365 and Exchange has a feature that automatically detects duplicate emails and prevents them from being re-delivered. This sometimes causes messages that are purposely re-delivered from the quarantine to not be delivered to the end user again.

The following article from Microsoft may help resolve this:

How to Configure Duplicate Message Detection in Exchange 2007 (External link)

The above article is for Exchange 2007, but may be helpful in finding the solution for other versions of Exchange. For Office 365, it is unknown if a solution exists. The following discussion at SpiceWorks may also help solve the issue:

Changing the Duplicate Detection setting in Exchange

The following registry setting allows you to change the number of hours that Exchange will keep track of duplicate deliveries (value is hours):

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\<Server Name>\<Private/Public-Guid>\Track Duplicates
Credit for this tip comes from:

Microsoft has been re-branding several of their products. This article should also apply to Exchange Online and Office 365.

Last updated May 9, 2023