If a Sales Order already has an email but it still asks to enter one, the cause of the problem is due to the missing email on a Drop Ship or Special Order items' preferred vendor. An accounting preference called Automatically Email Drop Ship P.O.s requires that an email is entered in the vendor record. Upon approval of a Sales Order, the system creates a Purchase Order. Then, the Purchase Order attempts to email it to the preferred vendor. This is why an email is required.
To resolve this, please go to the item record and identify the preferred vendor. Then, edit vendor record and enter a valid email.