"Mail steamers" were steamships which carried the mail across waterways, such as across an ocean or between islands, primarily during the 19th century and early 20th century, when the cost of sending a letter was declining to the point an ordinary person could afford the cost of sending a letter across great distances.In addition to carrying mail, most mail steamers carried passengers or cargo since the revenue from the mail service, if any, was insufficient by itself to pay for the cost of its travel.