Mail of any kind for transport by the US Postal Service should be enclosed in an appropriate envelope or parcel and sealed. The types of enclosures determine the mailing classification.
The following are samples of non-mailables in envelopes:
In addition to jamming and/or damaging the mailing machines, these items can also cause serious injury to Mail Center employees. The following are also classified as non-mailables and may be returned to sender:
Envelopes and Cards if pieces are less than:
All foreign countries also impose various restrictions depending on country and item(s) being mailed. It is recommended that when mailing questionable items to US or foreign destinations, the mailer should call Postal Services for assistance.
Enclosures that are designed so that the address appears in a window should not be stapled to prevent movement of the address from window view. If the addressed enclosure does not properly fit the window, use an envelope without the window.