This has come up before. Seems in almost every episode the mail slot is there in the door. I have tried to keep track, and in at least one show, the handyman, the slot is gone, and George, the mailman walks in and hands the mail to Andy. In another show, I believe, Barney goes to the post office to get the mail. Simply another inconsistency. In the interest of our collective mental health, it may be best if we just stop following minutia. Or is it all too much fun?