According to a post on Reddit it seems that Exchange 2016 and Exchange 2019 on-premises are queueing email messages starting januari 1, 2022 at midnight (UTC).

Besides mail queueing you will also see EventID 1106 in the application event log stating “The FIP-FS Scan Process failed initialization. Error: 0x80004005. Error Details: Unspecified error”

This might be a bug is how the date is handles inside the scan engine, causing it to fail after midnight UTC on Januari 1, 2022 (or is it December 31, 2021).

As there is no fix from Microsoft yet, the workaround is to disable anti-malware scanning on all your Exchange servers and restart the Transport service:

CD $ExScripts
.\Disable-AntiMalwareScanning.ps1
Restart-Service MSExchangeTransport

Update 1 at 7pm GMT+1: There are scan engine updates that allow you to re-enable anti-malware scanning. I did, mailflow is still working, but I still see EventID 5300 “The FIP-FS Microsoft Scan Engine failed to load” as shown below.

Update 2 at 9PM GMT+1. Microsoft is aware of this issue and working on a solution. Check the Exchange team blog: https://techcommunity.microsoft.com/t5/exchange-team-blog/email-stuck-in-transport-queues/ba-p/3049447

To be continued….