In principle, a postal service can be private or public. Governments often place restrictions on private postal delivery systems. Since the mid 19th century national postal systems have generally been established as government monopolies with a fee on the article prepaid. Proof of payment is often in the form of adhesive postage stamps, but postage meters are also used for bulk mailing.

Postal systems often have functions other than sending letters. In some countries, the postal system also has some authority over telephone and telegraph systems. In others, postal systems allow for savings accounts and handling applications for passports.