A can or drum with positive closures (clips, soldered
tops, screw-on caps) is acceptable for mailing certain
items such as liquids or powders. Generally, a can or
drum with only friction closures (push-down types) is
not acceptable. Protruding devices (such as locking
rings) must be shielded or covered with padding to
prevent injury to Postal Service employees or damage
to other mail or mail processing equipment.