We need to use the metaphor of letters to help users understand the mechanism of continuously updating sending address and receiving address. Users understand that when sending letters from different places to a friend, different sending addresses need to be written. If a friend moves, a different receiving address is required.

Yea but its somewhat unnatural to think of updating a receiving address if you're not moving (in the same app, in this context). Maybe "one time address" is clear enough?

I think it's probably confusing because its not that the "address" is updated, its that the "to" and "from" are written in code. Imagine there's a table where people can drop letters. You write who it's to and who it's from on the envelope: To: Alex From: Em If it's yours you pick it up. You and your friend don't want others to know you put a note in the pile so you come up with a secret code you both recognize for each other. The code takes the number of letters you've sent combined with your name to make some gibberish so only you and your friend know who it's from and who it's for: To: code(10, Alex) From: code(10, Em)