There are several classes and subclasses of mail designated by the United States Postal Service. The class of a piece of mail determines how it will be processed.

First-Class Mail

As a general rule, if something can be mailed, it can be sent first-class: Letters, postcards, parcels, business reply mail, etc. First-class mail is considered to be 'private' mail, and cannot be inspected unless there is a reasonable suspicion that it is carrying illegal or dangerous items (e.g. drugs, a bomb, or white powder such as that used to transmit anthrax). In such an event, only an agent of U.S. Postal Inspection Service (a separate law enforcement organization) or another law enforcement officer under Inspection Service authorization may open it.

Postage for any letters sent first-class mail weighing an ounce or less is a single 'letter' stamp (currently 37 cents). Businesses and other organizations which ship large quantities of first-class mail can also pre-pay the Post Office and show proof of postage with a metering impression. Additional postage must be paid for additional weight up to 13 ounces. All first-class mail weighing more than 13 ounces must be sent by Priority Mail. Postage for postcards is slightly less than for letters, the price is currently 23 cents.

First-class mail will be forwarded to the addressee's new address for 12 months after a forward request has been received. After this period is over, there is a 6-month period in which the Postal Service will return the mail to the sender with the new address.

First-class mail normally takes one to three days to reach domestic destinations, depending on the distance. Postal regulations require that all first-class mail received and processed by a post office before a mail carrier begins his/her route must be delivered that day.

Priority Mail

Priority Mail is simply any first-class mail over 13 ounces in weight. Rates for Priority Mail are higher than for regular first-class mail, starting at $3.85 for any item up to one pound. The maximum weight for Priority Mail (or any mail, for that matter) is 70 pounds, and its length and girth must be 108 inches or less. The rate for any item over six pounds is determined by a combination of weight and distance.

Global Priority Mail

This is a special international mail service which provides fast mail delivery to Canada and some nations in Europe and the Pacific Rim. All Global Priority Mail should be received within four business days of being sent.

Express Mail

A special service providing very rapid domestic delivery. Any item sent by Express Mail will be delivered by noon the next day unless otherwise stated in contract. The postage of any Express Mail item not delivered on time will be refunded (unless the recipient was not present for delivery). Express Mail rates are very expensive, beginning at $13.65 for items weighing eight ounces or less.

Periodical Class

Also known as second-class mail. Magazines, newspapers and other similar printed matter are sent by this class. Rates differ mostly depending on weight and frequency of publication. Only authorized companies and organizations may send mail at this class; it is not available to the general public. There are several other criteria required in order for an item to be sent at Periodical Class. It must:
  • be distributed at least four times a year
  • have paid subscribers
  • bear the name of the publication on the front
  • contain postage information in the first five or last five pages of the publication

    Like first-class mail, periodicals must be delivered the day they are processed by the local post office. Periodicals may be forwarded for up to 60 days.

    Standard Mail

    Also known as third-class. Standard mail can be any item weighing a pound or less that is not required to be sent first-class. Since it is considered 'non-private', Standard mail must be packaged in a way that is conducive to inspection for content and postage. It can be mailed either singly or by "bulk rates". Standard mail is most often used by companies to distribute flyers, catalogs, and the like; or to mail small merchandise or product samples. However, anyone may send mail by Standard class if the item meets the above requirements. Standard mail sent bulk rate must have the words "Bulk Rate" or "Nonprofit Organization" printed on the front. Standard mail sent singly must be labeled "Standard" or "STD" (don't ask who came up with that one.)

    Standard mail may be delivered up to three days after being processed at the receiving post office1. Standard mail is not forwarded unless the sender requests it. Indication of this is an endorsement on the item with phrases such as "Forwarding Service Requested", "Change Service Requested", etc. These items are forwarded at the expense of the sender. All undeliverable Standard mail not endorsed in this manner is destroyed.

    1: The one exception to this is political mail. This is due to claims from time to time by political candidates that they would have won an election if their ads had been delivered in time.

    Package Services

    Also known as fourth-class or Standard Mail (B), though the latter is no longer officially used by the Postal Service. The same rules that apply to Standard Mail apply to Package Service mail; the primary difference being that the latter is used for parcels and other large items.

    Bound Printed Matter: Can weigh no more than ten pounds.

    Media Mail: As the name suggests, this is any fourth-class mail consisting of audiovisual media; e.g. compact discs, films, etc. I believe (but am not sure) computer software can now also be sent as media mail.

    Library Rate: Again, the name pretty much says it all. Library Rate can only be used for packages sent to or from libraries.

    Parcel Post: All other Package Service mail.

    sid says Any first class package over one pound "is determined by a combination of weight and distance." It was over five, but that changed after the last rate hike. Also, the USPS did away with "Bulk Rate" almost a year ago. No Bulk Mail Unit will accept mail with "Bulk Rate" on the indicia. It's all STD or Non-Profit Org.
    I'm keeping this footnote here until I can confirm this; if it turns out to be correct, I'll edit the writeup accordingly.
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