SOLVED: Outlook saves .eml files with attachments to Inbox when opened

SOLVED: Outlook saves .eml files with attachments to Inbox when opened (Outlook 2016, Microsoft 365 Apps for Enterprise) 

Problem: Outlook saves .eml files with attachments to Inbox when opened

You may also get this message:

The properties of the message have been changed. Want to save your changes to this message.

Troubleshooting: Disable all non-Microsoft Outlook add-ins

Disable any non-Microsoft Outlook add-ins

Open Outlook
File - Options - Add-ins - Manage: COM Add-ins - Go

Untick all the Add-ins except these:
Microsoft Exchange Add-in
Microsoft Teams Meeting Add-in

Close and re-open Outlook and see if the issue still exists.
Then, re-enable the Add-ins one by one to try and identify the one that is causing the problem.

Solution: Disable Adobe Outlook Add-ins and ESET Antivirus Outlook Add-in

Untick these Add-ins:
Adobe PDFMaker Office COM Addin
Adobe Document Cloud for Microsoft Outlook

This issue can also occur when the ESET Endpoint Protection Outlook Add-in is enabled

Untick this Add-in:
ESET Outlook Add-in