The type of letters that are delivered through the normal mail or by airmail express service is referred to as
Answer Details
The type of letters that are delivered through the normal mail or by airmail express service are called airmail letters. These are letters that are sent through the postal service and are delivered by air instead of ground transportation. This type of service is usually faster than regular mail and is often used for sending letters or packages to other countries. The letters are typically marked with an "airmail" label to indicate that they should be delivered by air.