Gave it a try. Whole concept seems pretty cool and useful. In-fact, I acquired English as my second language in a kind of similar fashion which this app suggests. I used to write emails, send texts to my English speaking pen-pals. In that way I learned a lot of expressions.
However, the one problem with this app is that the letters are AI generated if I am not wrong. It would have been better if the app had organic users writing emails to each other besides the AI generated stuffs.
Stamper
Cash Flow Portal
BlissBox