Frankly, there is d--n all you can do about it.
When we lived at no. 127 in a block of flats that were no. 127, 127a, 127b, 127c, 127d and 127e anything and everything addressed to the first floor landed on our doormat, irrespective of what letter was written after the number or the name of the person the mail was addressed to.
When we complained, the postmaster said the postmen read the floor number, as it isn't their business to keep track of who lives where. I pointed out that the names of all three people living in the flat where on the door and that when sorting post surely they could check the presence or absence of a letter after the number, but no, they couldn't or wouldn't.
I agree Mr. and Mrs. X should complain that they are not getting their post.