Outlook 365 client has started opening some emails in resizable windows that take up the entire screen. Behavior is intermittent.

Hi Will, 

Good day. Thank you very much for sharing your experience and providing the details of your situation. 

Just a quick note before we continue: this is a user-to-user community forum, so I don’t have access to your Microsoft 365 tenant, your account, or your device to make changes directly. However, I’m here to support you the best I can within these constraints, by providing clear next steps, sharing applicable resources, and directing you to the appropriate support channels.  

Based on your description, the issue appears to be related to how Classic Outlook handles the saved size and position of email message windows. Normally, when you double-click an email and open it in a separate window, Outlook should reuse the size of the most recently closed message window. 

Since this behavior started after a recent update and does not occur every time, it seems likely that the issue may be related to the current Outlook build you are using, Version 2605 Build 16.0.20026.20166. 

As a next step, I would suggest updating Outlook/Microsoft 365 Apps to the latest available build, Version 2605 Build 20026.20182, to ensure Outlook has the most recent fixes and improvements applied.

To update Outlook, you can follow the steps below: 

1. Open Outlook. 
2. Go to File > Office Account. 
3. Under Update Options, choose Update Now.   (If the Update Now option is not available, Office updates may be managed or disabled by your IT administrator.) 
4. Allow Office to complete the update process. 
5. Once the update is finished, restart Outlook. 
6. Then, test the behavior again. 

References: Update history for Microsoft 365 Apps (listed by date) - Office release notes | Microsoft Learn 

If the issue still continues after updating, please try opening Outlook in Safe Mode to check whether any add-ins may be affecting the behavior: 

1. Close Outlook. 
2. Press Windows + R. 
3. Type:  outlook.exe /safe 
4. Click OK, then test opening a few emails again. 

When the issue does not occur in Safe Mode, please disable Outlook add-ins by going to: 

File > Options > Add-ins > Manage COM Add-ins > Go, then uncheck non-essential add-ins, restart Outlook, and test again. 

Should the issue persist even after these steps, it may stem from a deeper system-level cause. In that case, I would recommend reaching out to your organization's IT and asking them to open a support ticket with Microsoft Support through the Microsoft 365 Admin center so the issue can be investigated further with logs and environment details. 

In case you do not know who is your IT admin, kindly refer to this article: How do I find my Microsoft 365 admin? - Microsoft Support   

I hope this information is helpful. Should you have any further questions or need additional assistance, please feel free to share them in the comment below. I'm very happy to help.  

Thank you again for your patience and understanding. 

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