Mail in ballots will be delivered by members of the Postal Union which has come out forcefully for Biden. Mailmen sometimes dump mail if they feel they are overworked or for other reasons. I expect they will all have maps or lists of known Republican neighborhoods and enclaves and such mail can be short circuited or mistakenly delivered o Democrat Party hqs for further processing.
Unlikely in the extreme. I doubt even the federal government has demographic information as fine-grained as that.