Microsoft said it has fixed the bug that was causing emails to get stuck in the transport queues of on-premise Exchange Server 2016 and Exchange Server 2019 and disrupting the services of several enterprise customers.
Microsoft's Exchange team clarified that the issue was not related to security and there was no issue with malware engine or malware scanning as some users felt.
The bug was triggered on 1 January by a date check failure after the change of the year from 2021 to 2022.
"The version checking performed against the signature file is causing the malware engine to crash, resulting in messages being stuck in transport queues," the Exchange team said in a blog post.
The disruption has created a backlog of emails. The Exchange team said the time to clear the backlog will vary from customer to customer and will depend on the volume of emails stuck in the queues.
The team urged customers to run the "reset scan engine version" script on each Exchange mailbox server that is used to download anti-malware updates in their organization.
Customers can also fix the issue manually by deleting existing malware engines and metadata and updating to the latest engine.
To avoid the disruption of email service, many enterprise customers chose to bypass the anti-malware engine by disabling it. According to the Exchange team, those customers can enable it now after running the script released by Microsoft.
Exchange Server 2019 is going to be the last on-premise product. Microsoft is phasing it out in favour of the new Exchange Server vNext, under which customers will be moved from the existing licensing model to a monthly/yearly subscription fee.