The "Viewing" was changed back after I applied changes
macOS High Sierra email problem - The "Viewing" was changed back to the new default setting (even I applied the changes).
How to disable those Default "Viewing" setting in "Mail" in High Serra?
macOS High Sierra email problem - The "Viewing" was changed back to the new default setting (even I applied the changes).
How to disable those Default "Viewing" setting in "Mail" in High Serra?
See some articles View emails as conversations - Apple Support
Mail Viewing preferences - Apple Support
If you have created filters in mail , you can disable filters Filter emails - Apple Support
The "Viewing" was changed back after I applied changes