Millions of people did not receive emails as the bug prevented on-premise Microsoft Exchange servers from sending emails.
According to Microsoft, the problem related to a date check failure with the change of New Year.
"It is not a failure of the AV engine itself. This is not an issue with malware scanning or the malware engine, and it is not a security-related issue," Microsoft said in a security update.
"The version checking performed against the signature file is causing the malware engine to crash, resulting in messages being stuck in transport queues," the company explained.
Microsoft has now addressed the issue causing messages to be stuck in transport queues of on-premises Exchange Server 2016 and Exchange Server 2019.
"We have now created a solution to address the problem of messages stuck in transport queues on Exchange Server 2016 and Exchange Server 2019 because of a latent date issue in a signature file used by the malware scanning engine within Exchange Server," said the company.
The newly updated scanning engine is fully supported by Microsoft.