Pillar boxes on the island of Madeira. (1st class mail in blue and 2nd class in red)

Letters

Letter-sized mail comprises the bulk of the contents sent through most postal services. These are usually documents printed on A4 (210×297 mm), Letter-sized (8.5×11 inches), or smaller paper and placed in envelopes.

While many things are sent through the mail, interpersonal letters are often thought of first in reference to postal systems. Handwritten correspondence, while once a major means of communications between distant people, is now used less frequently due to the advent of more immediate means of communication, such as the telephone or e-mail. Traditional letters, however, are often considered to harken back to a "simpler time" and are still used when someone wishes to be deliberate and thoughtful about his or her communication.

Bills and invoices are often sent through the mail, like regular billing correspondence from utility companies and other service providers. These letters often contain a self-addressed, envelope that allows the receiver to remit payment back to the company easily. While still very common, many people now opt to use online bill payment services, which eliminate the need to receive bills through the mail.

Bulk mail is mail that is prepared for bulk mailing, often by presorting, and processing at reduced rates. It is often used in direct marketing and other advertising mail, although it has other uses as well. The senders of these messages sometimes purchase lists of addresses (which are sometimes targeted towards certain demographics) and then send letters advertising their product or service to all recipients. Other times, commercial solicitations are sent by local companies advertising local products, like a restaurant delivery service advertising to their delivery area or a retail store sending their weekly advertising circular to a general area. Bulk mail is also often sent to companies' existing subscriber bases, advertising new products or services.

There are a number of other things almost without any exception sent exclusively as letters through postal services, like wedding invitations.

First-class

First-class mail in the U.S. includes postcards, letters, large envelopes (flats) and small packages, providing each piece weighs 13 ounces or less. Delivery is given priority over second-class (newspapers and magazines), third class (bulk advertisements), and fourth-class mail (books and media packages.) First-class mail prices are based on both the shape and weight of the item being mailed. Pieces over 13 ounces can be sent as Priority Mail. In the U.K., First Class letters are simply a priority option over Second Class, at a slightly higher cost. Royal Mail aim to (but do not guarantee to) deliver all First Class letters the day after postage.

Registered and recorded mail

Registered mail allows the location and in particular the correct delivery of a letter to be tracked. It is usually considerably more expensive than regular mail, and is typically used for legal documents, to obtain a proof of delivery.

Repositionable Notes

The United States Postal Service introduced a test allowing "repositionable notes" (for example, 3M's Post-it notes) to be attached to the outside of envelopes and bulk mailings, afterwards extending the test for an unspecified period.

Postal cards and postcards

Postal cards and postcards are small message cards which are sent by mail unenveloped; the distinction often, though not invariably and reliably, drawn between them is that "postal cards" are issued by the postal authority or entity with the "postal indica" (or "stamp") preprinted on them, while postcards are privately issued and require affixing an adhesive stamp (though there have been some cases of a postal authority's issuing non-stamped postcards). Postcards are often printed to promote tourism, with pictures of resorts, tourist attractions or humorous messages on the front and allowing for a short message from the sender to be written on the back. The postage required for postcards is generally less than postage required for standard letters; however, certain technicalities such as their being oversized or having cut-outs may result in payment of the first-class rate being required.

Postcards are also used by magazines for new subscriptions. Inside many magazines are postage-paid subscription cards that a reader can fill out and mail back to the publishing company to be billed for a subscription to the magazine. In this fashion, magazines also use postcards for other purposes, including reader surveys, contests or information requests.

Postcards are sometimes sent by charities to their members with a message to be signed and sent to a politician (e.g. to promote fair trade or third world debt cancellation).

Other

This antique "letter-box" style U.S. mailbox is both on display and in use at the Smithsonian Institution Building.

Larger envelopes are also sent through the mail. These are often made of sturdier material than standard envelopes and are often used by businesses to transport documents that are not to be folded or damaged, such as legal documents and contracts. Due to their size, larger envelopes are sometimes charged additional postage.

Packages are often sent through some postal services, usually requiring additional postage than an average letter or postcard. Many postal services have limits on what can and cannot be sent inside packages, usually placing limits or bans on perishable, hazardous or flammable materials. Some hazardous materials in limited quantities may be shipped with appropriate markings and packaging, like an ORM-D label. Additionally, because of terrorism concerns, the U.S. Postal Service subjects their packages to various security tests, often scanning or x-raying packages for materials that might be found in mail bombs.

Newspapers and magazines are also sent through postal services. Many magazines are simply placed in the mail normally (but in the U.S., they are printed with a special bar code that acts as pre-paid postage - see POSTNET), but many are now shipped in shrinkwrap to protect the loose contents of the magazine.

Hybrid mail, sometimes referred to as L-mail, is the electronic lodgement of mail from the mail generator’s computer directly to a Postal Service provider. The Postal Service provider is then able to use electronic means to have the mail piece sorted, routed and physically produced at a site closest to the delivery point. It is a type of mail growing in popularity with some Post Office operations and individual businesses venturing into this market. In some countries, these services are available to print and deliver emails to those unable to receive email, such as the elderly or infirmed. Services provided by Hybrid mail providers are closely related to that of Mail forwarding service providers.