Hi @Wonderwhy
Yes, is your issue looks like this?
When you are replying an email messages in Outlook, the RE: Message window is embedded in the Reading Pane by default.
If you want to release the RE: Message window, just click the Pop Out button at the upper-left corner of embedded window.
And if you want to disable Replying In Reading Pane For All Email Messages,
Step 1: Click the File > Options.
Step 2: In the Outlook Options dialog box, click the Mail in the left bar.
Step 3: Go to the Replies and forwards section, and check the option of Open replies and forward in a new window.
Step 4: Click the OK button.
Refer to this link to get more information: How To Disable Replying In Reading Pane In Outlook?
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