When sending internal emails with Office attachments (.docx / .xslx) the email gets blocked and moved to quarantine. The Administrator gets notified that the file is a broken archive.
This issue is caused by an external antivirus (usually Windows Defender) incorrectly identifying the attachments as viruses and making them inaccessible to MailEssentials. The Decompression Engine will not be able to inspect the attachments and that causes the email to fail.
In order to prevent this unwanted behavior, please follow the steps described in the Recommended Anti-virus and Backup Exclusions article.
After excluding the folders from the external antivirus, internal emails with attachments are processed correctly.