If you sent a certified or priority letter, you can come back to your account to view tracking information for that document. To get this, locate the job in your sent items and click it. Tracking information will be in a green box below the From section of the message details. The tracking number is located below the production status line. You can click the "Get Tracking" button to get additional tracking information and current status.
I've drawn a box below the name to hide the address of this mail recipient in the image below, but it will be present when you view a message in your account.