Postal Explorer > Publication 52 - Hazardous, Restricted, and Perishable Mail > 6 International Mail > 63 Restricted Matter: International Mail
Mailable restricted matter sent from, to, or between overseas military post offices (APOs/FPOs) must meet the requirements for international mail because military mail is often diverted to air transportation when space is available or when surface transportation is not available. See DMM 703.2.