Unsealed mail can result in many problems, including content damage, security, processing delays and loss of mail, states the United States Postal Service website. This in turn leads to problems for the postal process such as jams, diverter issues and double feeds, resulting in lower performance of the mail service.
However, USPS offers an unsealed mail category for mail that doesn't fall into the more common category of mail that is sealed and not subject to inspection.
The cases in which the Postal Service allows unsealed mail include periodicals such as magazines or newspapers, standard mail, certain Package Services items, certain domestic mail enclosures or attachments, Global Express Guaranteed items not including documents, airmail M-bags for large print shipments, items sent for blind and physically handicapped persons, and some Global Direct mail. More information on specific Global Direct mail categories can be found on the USPS website.
In cases of Priority Mail for international items, two unsealed envelopes are permitted, with the exception of flat-rate envelopes and the small flat-rate box containers.
If the envelope being sent doesn't fall under any of the above mentioned categories, the USPS recommends ensuring that the envelope is sealed before mailing. Sealing an envelope is just as important as using the right postage and correct address, says a spokesperson for the USPS.