Dear Schoedel, Daniel,
Thank you for providing the troubleshooting steps you've already completed.
Since the message only appears when using HTML format and does not occur when the default message format is changed to Rich Text, this suggests that Outlook may be attempting to load an HTML-related resource, such as a stationery file, theme, image, or other web content that is no longer available or cannot be accessed.
Firstly, to help narrow down the cause, could you please clarify a few points?
- Is the issue occurring only for this user, or do other users experience the same behavior?
- If you create a brand-new blank HTML email containing only plain text, does the warning still appear?
- Does the issue follow the user when signing into Outlook on another computer?
- Does the same issue occur when creating a new email in Outlook on the web (OWA)?
- Are other users in your organization experiencing the same issue?
As additional troubleshooting, please try the following:
Step 1: Start Outlook in Safe Mode
Right-click the Start button and click Run > type: Outlook.exe /safe and test creating a new email.
Step 2: Disable all COM Add-ins
Outlook > File > Options > Add-ins > Manage: COM Add-ins > Go > Uncheck all add-ins and restart Outlook
Step 3:
Check File > Options > Mail > Stationery and Fonts and ensure that no custom stationery, theme, or background image is configured.
Also check: Right-click the Start button and click Run > type: %APPDATA%\Microsoft\Stationery > Rename the folder temporarily and test again.
Step 4: Check Internet Explorer / Edge Legacy HTML Settings
Open: Control Panel > Internet Options > Under: General > Browsing History > Delete > Clear: Temporary Internet Files and Cached Web Content > Then reset Internet Options to default settings.
Step 5: Repair the Outlook app
Go to Windows Settings > Apps > Installed apps > Outlook > ... > Advanced options > Repair. Otherwise, you can go to Control Panel > Programs and Features > Select Microsoft 365 Apps > Change > Online Repair
Moreover, if possible, according to my research, I recommend you try these:
- Email is set to HTML
- Changed the PNGs to JPGs
- Kept only one image and deleted the rest
- Removed the table layout template that Microsoft supplied
- Removed all content originating from the Microsoft template.
Please try these steps and let me know the result. If you have any questions or if I have misunderstood anything, please feel free to reach out. I'm looking forward to your update.