I spent months testing and trying this out with some doors i made until i found everything i could that would cause it, unless something new has appeared it's almost definitely one of three things:
1. It may be that the edge of the door may be too close to the lip of the mailbox and is very slightly clipping the lip as it moves, that would need to be changed in your modeling program.
2. It is place so that the animated part of he object is clipping another entity nearby while animating (with one of my doors i had made the doorway so tight that although when closed and open it was clipping, when it rotated the trailing edge of the door was passing through the door frame), moving it away from the other entity to avoid clipping fixes this (i think grass clipping the animated object can do this too).
3. The most common cause is literally having the bottom of the model exactly level with the origin point, this means that it's on the same level as the terrain mesh and thus clipping, if you place it then just raise it slightly up so it's not touching the terrain and try it again that would fix that, you would want to slightly raise the model above its origin point in the modelling program for a more permanent solution.
i5, NV960 2GB, 16GB memory, 2x 2TB Hybrid, Win10.
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Intel Celeron (duel Core), Radeon integrated graphics, 4GB memory, 180gB Generic SATAII, WinVista.
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