What is the safest, cheapest legal way to send money to Bangladesh from the UAE?

Use a licensed exchange house or bank app, not hundi. Hundi is illegal and gives no protection if the money is lost. Licensed exchange houses in the UAE include Al Ansari Exchange, LuLu Exchange and GCC Exchange; they give a receipt. You need your Emirates ID to send money. The Bangladesh government adds a 2.5% cash incentive on money sent through legal channels, paid to your family in Taka. On BDT 100,000 they get an extra BDT 2,500. For amounts over BDT 500,000 documents are needed. Compare the fee and exchange rate before you send, and keep your receipt.