The macOS Catalina MAIL does not have the function to NOT open an email. ie scroll up from the bottom of page to see the origin of sender. Who is joining me in telling Apple that this a bad move because one has to open SPAM by default.
MacBook Pro 15", macOS 10.15
In this way you are not really opening anything just "previewing" the email.
There is no harm there.
If Junk mail is not working for you as expected you can turn it off in Mail>Preferences.
You have the option of installing a third party spam filter. ex.
https://c-command.com/spamsieve/
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Junk mail not working
https://support.apple.com/guide/mail/if-junk-mail-filters-arent-working-mlhlp1145/mac
https://support.apple.com/guide/mail/junk-mail-preferences-cpmlprefjunk/mac
https://support.apple.com/guide/mail/reduce-junk-mail-mlhlp1065/mac
Identify and filter junk mail in iCloud - Apple Support
https://support.apple.com/en-in/guide/mail/junk-mail-advanced-settings-cpmlprefjunkadd/mac
https://support.apple.com/en-in/guide/mail/viewing-preferences-cpmlprefview/mac
Mail advanced junk mail settings on Mac - Apple Support
https://support.apple.com/en-in/guide/mail/junk-mail-advanced-settings-cpmlprefjunkadd/mac
