Hi there,
It seems It's a bug that others are also experiencing on some emails in Windows Mail app. If you have recently updated your PC try to uninstall the update and wait still this issue is fixed. Meanwhile you can try to send the mail from your Web browser or other web clients. Meanwhile you can raise this issue in the feedback hub.
The Feedback Hub app lets you tell Microsoft about any problems you run into while using Windows 10. You can also send suggestions to help us improve your Windows experience. https://support.microsoft.com/en-us/windows/send-feedback-to-microsoft-with-the-feedback-hub-app-f59187f8-8739-22d6-ba93-f66612949332
You can keep a track of these issues by following this article. Windows 11 known issues and notifications https://learn.microsoft.com/en-us/windows/release-health/status-windows-11-21h2
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