The outbox-to-inbox ratio is basically the balance of how many emails you’re sending versus how many you are receiving, and it too can hurt the reputation of your mailbox. Our advice is that you try to keep the ratio below 4, which translates to 4 emails sent for 1 email received.
This is a historical ratio, based on the number of emails exchanged by your mailbox since it's creation.
Try hovering your mouse over the ratio to see the actual number of sent and received emails.