You don't need to send them anywhere. You actually can take the empties to any location and exchange them for a new lippie (except the Viva Glam ones).
Here is more info from the site:
Back to M·A·C Program
As a part of a pro-environment commitment to recycle/reuse waste, M·A·C recycles its primary packaging through the Back to M·A·C Program. M·A·C encourages our customers to return primary packaging of many M·A·C products to M·A·C counters. This packaging is then forwarded to appropriate recycling centers. By returning six (6) M·A·C primary packaging containers to a M·A·C counter, you will receive a free M·A·C Lipstick of your choice [except for the Viva Glams]. This is how we thank you for helping the environment through recycling.
Most primary packaging of M·A·C products is accepted as Back to M·A·C "currency." M·A·C does not accept the following as a part of the program:
* Secondary packaging of any sort [paper box, plastic/cellophane wrapper, shopping bags, etc.]
* Samples [sample containers, special/trial size products for promotional/seasonal kits]
* Cosmetic applicators/tools [disposables, puffs, sponges, scissors, spatulas, swabs, sharpeners, tweezers, curlers]
* Accessories [bags, belts, makeup cases, lashes, Blot Film]
In addition to our "Back to M·A·C Program," M·A·C uses minimal packaging and recycled paper to paperboard packaging in our offices, manufacturing and distribution facilities. Vegetable-based inks are used for printing whenever possible. At M·A·C we share your concerns about the environment and we are continually looking for ways to address those concerns while providing our customers with complete product satisfaction and safety.