If you’ve got email that’s more than a year old just clogging up your Inbox (and taking up valuable space), you can use a Smart Mailbox to help you do some fast email house cleaning.
Just Control-click on the email account (or your Inbox if you don’t have multiple accounts) that you want to clean up, and then choose New Smart Mailbox from the contextual menu. When the Smart Mailbox dialog appears, from the first criteria pop-up menu on the left, choose Date Received. From the next pop-up menu over, choose “is before the date,” and in the final field, type a date that is approximately one year before today. Click OK and all your email that is one year old (or older) will appear in that Smart Mailbox. To delete that old email, just click on the Smart Mailbox, press Command-A to select all the email, then press the Delete key on your keyboard.
Now, the nice thing is that tomorrow more one-year-old email will appear in that Smart Mailbox (thanks to its live updating), and the next day, and the next day, and so on, so your mailbox never has more than one year of archived messages. So, about once a month, click on that Smart Mailbox and easily delete all the old email.