Set Delay on Mark Mail as Read in Mac Mail
Close Mac Mail, and open Terminal and copy paste in the command below, eding the 0 to the number of seconds of delay you’d prefer, like 5 or 30. Hit enter and you’re done!
defaults write com.apple.Mail MarkAsReadDelay 0
To go back to the default, mark read immediately:
defaults delete com.apple.Mail MarkAsReadDelay 0