How to Send a Package to Another Player
Preparing the Package
Craft a Package using the mod's crafting recipes. A Package is the container you fill and send. Once you have a Package, place the items you want to deliver inside it. The Package holds whatever you put in before sealing.
Sealing and Stamping
Seal the Package with Packing Tape. The Packing Tape closes the Package so its contents are locked in for delivery. After sealing, apply a Stamp to the Package. The Stamp is what marks the Package for delivery through the Ender Mail system. Without a Stamp, the Package will not be sent.
Delivering
With a sealed, stamped Package, send it to the target player. The Package is routed to that player's Locker. The recipient opens their Locker to collect it. Make sure the recipient has a Locker placed, or the delivery has nowhere to land. If you run a Package Controller, use it to manage outgoing Packages from your base instead of handling each one by hand.