Between 1970 and 2017, 548 terrorist attacks involved explosives in letters, parcels, or packages sent in the mail or made to appear as if they were sent in the mail.
Distler, Michael, and Erin Miller. 2018. "Terrorist Attacks Involving Package Bombs, 1970 — 2017," START, College Park, Maryland. October. www.start.umd.edu/pubs/START_PackageBombs_FactSheet_Oct2018.pdf