Mail of any kind for transport by the United States Postal Service should be enclosed in an appropriate envelope or parcel and sealed. The types of enclosures determine the mailing classification.

Non-Mailable Material

The following are examples of materials which should not be mailed in envelopes:

  • Pens and Pencils
  • Paper Clips
  • Glass Chips
  • Staples
  • Metal Pieces
  • Sand

In general, any material which causes a bulge should not be mailed in an envelope. Such materials will not only jam or damage the mailing machines, but can also cause serious injury to Mail Services employees.
Foreign countries often have unique restrictions regarding enclosures. Check with the Mail Services Supervisor if in doubt.
In addition, the following are also classified as non-mailable and may be returned to sender:
Envelopes and Cards if these pieces are less than:

  • 3 1/2 inches in height
  • 5 inches in length
  • 007 inches in thickness
    (thickness of a post card)

Window Envelope Enclosures

Enclosures, designed so that the address appears in a window envelope, must not be stapled to prevent slippage of the address from window view. If the address enclosure does not properly fit the window, use an envelope without a window. Please do not staple mail into envelopes.


Appropriate Size and Strength

Enclosures will determine the appropriate envelope to be used, meaning the envelope will be one of the right size and strength.

The size should properly accommodate the insert(s). Excessively large envelopes will not keep the inserts firm. The inserts in such envelopes slide about, create an imbalance of the envelope, risking ripping and loss of contents. A snug fit keeps the enclosure firm in the envelope providing for effective mail handling.

Conversely, when an envelope is overstuffed it can burst at the seams when machine processed. The result can be a total loss of the mailing.

The strength of the envelope should be such to withstand the weight of its contents. If there is stress at the seams or sharp edges, the envelope is in an overload condition where it can burst or tear apart and lose its contents.