When you drag a plist out of its parent folder, a new, pristine plist is created to replace it (that could explain your observation). Also, you should drag the plist out while Mail is not open. Try resetting your Mail preferences and see if they stick.
Jon
macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365
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