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After testing on my Outlook 365 (Classic), this issue may be related to the zoom level in the message window.
Please check the Zoom slider at the bottom-right corner of the window, or hold Ctrl and scroll up with your mouse wheel to increase the view size.
If you open the email in a separate (pop‑out) window, the Zoom slider will not be available, but you can still use Ctrl + Scroll Up to zoom in on the message content.
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