If you are handling a busy inbox with swathes of emails, one device to make it easier to deal with is to create a
Smart Mailbox. Smart mailboxes don't file your emails, but give you a filtered view of what they contain. Much better than filing, as many emails can't be accurately filed under just one heading. Fortunately, smart mailboxes 'look' in every mailbox, so it doesn't matter if you have already filed things, they still get found.
To create a Today smart mailbox, go to
Mailbox... New Smart Mailbox and set it up similar to this, by choosing from the popup criteria, and then adding extra criteria to filter more precisely. Name the mailbox"Today" and click OK.
Your results should look similar to mine. You can right-click on the Today smart mailbox to fine-tune the criteria until it shows just what you need.
You can have Mail display the result in a nice compact window by choosing
View... Hide Mailboxes (you can customise your view and have this as a button if preferred) and then resize the window.
Remember too that you can have multiple Message viewing windows open, and they can display different smart mailboxes (say, emails from an important client). Your mailbox settings are retained after quitting, for the next session.