Customizing Search Result Display for .eml files at Site Collection level

No, in modern SharePoint Online you cannot change the default search result display template or override managed property mapping at the site collection level. The behavior you’re seeing with .eml files (GUID filename shown instead of the Title) is expected and cannot be modified.

Modern SharePoint search uses a fixed, tenant‑controlled display template:

• It always prioritizes FileName (FileLeafRef) as the main label.
• Even if the Title property (email subject) is populated, modern search does not allow you to switch the display to Title at the site level.
• Site Collection Admins can only adjust mappings for custom managed properties; not system properties like Title, Filename, or the built‑in display logic.

You can use a Classic Search Center and configure a custom display template that:

• Uses Title as the primary heading
• Falls back to Filename only if Title is empty

However, I do not recommend classic search approach. It changes the overall user experience, requires a separate Classic Search site and additional navigation links in your existing site, and introduces extra training and change‑management challenges. Users would need to go to a different site or page to perform their searches.

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