Any required address, marking, or endorsement placed on the outside of a
mailing container should be made with a waterproof material that cannot be
rubbed off or smeared. The lettering must be sharp and readable at a
distance of 30 inches.
Marking methods and container surfaces must allow application and retention
of adhesive stamps, postage meter impressions, and postal endorsements
made by hand stamp, ballpoint pen, or number 2 pencil. Address labels and,
particularly, outside envelopes must be firmly sealed and attached to
containers, with no more than 1/8-inch separation between the ends of the
envelope and the container.
A container that does not meet these marking requirements and has caused
previous mail processing problems is not acceptable for mailing.