One possible explanation for this behavior could be related to the settings within the Mail app itself. It's possible that a setting may have been inadvertently changed, causing emails to behave differently than usual.
To address this issue, you can start by checking the preferences settings within the Mail app. Specifically, look for settings related to message handling or message preview. There might be an option that controls whether emails open with a single click or require a double-click to open. Adjusting this setting to your preference should resolve the issue.
If you're unable to locate the specific setting causing the problem, you can also try resetting the Mail app to its default settings. This can often resolve issues related to unexpected behavior. To do this, you can go to the "Preferences" menu in the Mail app and look for an option to reset or restore default settings.
If the issue persists despite these steps, it may be worth reaching out to Official Apple Support for further assistance. They can provide personalized support and may be able to offer additional guidance or troubleshooting steps tailored to your situation.