Our system begins processing Bill Pay items 2-5 days before the date that payments are scheduled to be delivered. Payees who can receive electronic payments have shorter lead times than those who only accept paper checks that are sent by mail. You can see how your payment will be sent by looking for the icon next to each payee. If you see a square shaped icon with a lightning bolt, that indicates the payee will receive their payment electronically. A rectangular check icon indicates the payee only receives payments via mail.
Bonus tip: Consider giving yourself a bit a leeway on timing with mailed payments, since delivery times from the postal service can vary.