+1 on GM's answer, sense in two axes.
I see you just posted and caught both of these next two ideas but I'll leave them there anyway. The new idea you just added, the only prob with the one sensor at the front is that you have to continuously-forever run the sensor, and catch one event day that lasts only a second or two. The multiple sensors can glance casually only once in a while.
Sensors at the center of the box, at 6" depth from the front both horizontal and vertical, might not see a piece of mail that is too short, and placed at either the very front or the very back of the box. I'd place sensors at the 4" and 8" depths, again in both horizontal and vertical dimensions.
That's 4 rather than 2 sensors which is already twice as many as 1 sensor. IR receivers are extravagantly expensive. Plain LED and photocell are dirt cheap. I this outdoors? Probably not if there's no door. Sun can interfere with both photocell and IR, but if not directly facing prevailing sun shouldn't be a problem.
You could also teach Schrodinger's cat to fetch the mail. Until the mail is fetched it can be thought of as both real mail and junk mail at the same time, and it's not worth the effort of getting the junk mail till there's also real mail in the box.