IDENTIFYING INCENDIARY AND EXPLOSIVE MAIL BOMBS Incendiary and explosive bombs have been contained in and disguised as letters, books, and parcels. Following are 10 of the most obvious indications of a mail bomb: letters or packages that appear rigid or excessively bulky. wrappers marked by oily stains. packages with an excessive amount of postage. omission of the sender’s name and address. lettering made from cut-out or poster letters instead of being handwritten or typed. package emitting a peculiar odor. package with string or wire visible. package or letter that is unusually resistant to being opened. package showing soft spots, bulges, or previous traces of glue or tape. package of irregular shape.