If you stop leasing a domain, you should be very sure that you changed your e-mail address at all the services you ever used and informed all the contacts of your new e-mail address. Because the second you let your domain expire, someone else might gobble it up and receive all the e-mail addressed to you. If you keep leasing for multiple years even after migration, you can be sure to catch any services/people still mailing you at the old address.