Correspondence - Mail of any kind for transport by the USPS should be sealed in an appropriate envelope or parcel. The types of enclosures determine the mailing classification to be utilized.
- Items - Some items cannot be sent through the U.S. mail in envelopes. In addition to jamming and/or damaging mailing machinery, these items can potentially cause serious injury to Mail Services employees. Examples of items that cannot be mailed in envelopes include paper clips, metal pieces, glass chips and sand.
- Sizes - The following are classified as non-mailables due to size, and may be returned to sender:
- Envelopes and cards less than 3.5 inches in height.
- Envelopes and cards less than 5 inches in length.
- Envelopes and cards less than .007 inches in thickness (the thickness of a postcard).
NOTE: All foreign countries also impose restrictions, which vary with country and item(s) being mailed. It is recommended that when mailing questionable items to U.S. or foreign destinations, the mailer should call Mail Services for assistance.