Hi @Thea ，
Considering I cannot reproduce your issue in my Outlook client, what's the version of your Outlook? (File > Office Account > About Outlook). Please make sure you have upgraded your Outlook to the latest version.
I have a couple of users that can't open .msg files in Outlook.
How did you open the .msg files? By simply double clicking them? If yes, as a workaround, you could try to drag your .msg files into your Outlook desktop client and see if the issue has any difference.
The command line argument is not valid. Verify the switch you are using.
I did some search about this error message and found some related information, please try the suggestions below to see if it could help:
- Check your windows settings again and make sure your default Email app and default app for .msg file is Outlook desktop client.
- Windows Mail app sometimes might cause issues like this. If necessary, you could try to remove the Windows Mail app and restart your windows to see if this issue could be resolved.
- Corrupted windows .dll files and windows user-custom settings might cause similar issues as well. Before reinstalling the PCs, creating a new windows profile should be a simpler method to exclude part of those influence.
- If the issue still continues, as you mentioned, reinstalling the PCs completely should be the last workaround of this issue.
For more information, please check:
the command line argument is not valid. verify the switch you are using.
MS Office Outlook no default mail client error 365/2016
Hope this can be helpful.
If an Answer is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.