15 Summer Hill that lot is almost totally flat shouldn't be problems with it. Make sure the mailbox is at least 3 tiles away from the sidewalk and any other lot edge.
Only other thing I can think of is that something went funky when the world loaded the first time you started to play in it.
Also if you have Pets installed and removed the horse stables that the game places there automatically it leaves invisible spawners behind those could cause routing problems.
This only applies if you had the stables:
Go into buy mode ctl+shift+c type testingcheatsenabled true hit enter then ctl+shift+c buydebug on and click on the question mark and see if you can spot any spawners and delete them after those are gone ctl+shift+c buydebug off and then ctl+shift+c testingcheatsenabled false.