The difference between internal and external mail is the location of the intended recipient. Internal mail is communication via paper mail or email that is within a company. External mail is communication via mail or email that is distributed outside the company.
Internal and external mail are typically very different in tone, style and delivery methods. Internal communication in a business is less formal than correspondence that is sent outside the business. Internal communication may be between people who are in close proximity or opposite ends of a building. On the other hand, external mail is between people who are typically not in the same location.
The main purpose of both types of mail is to share information and ideas with the recipients. Internal mail is used to share information between one or more recipients. The exchange of internal mail may be very quick when email is used or somewhat slower when a courier is used.
External mail is most often addressed to a particular person and may not be an exchange of information. External mail is often used to keep the lines of communication open for business purposes. External mail is not as quick, and typically relies on the postal service. However, external mail may be in the form of email and thus may be as quick as internal mail.