Correct drainage setup for a wall-mounted AC?
First, the damage that can be caused is severe. Don't run the unit until this is fixed.  Here's the deal, these units are indeed installed on the wall, typically high up on the wall relatively near the ceiling.  The tubing package is designed to exit out of the back of the unit and down the outside of the wall the unit is installed on. Here is the key though:  As soon at this tubing package, (the refrigerant lines, the electrical line and the drain line)as soon as they exit  the back, at the very same time this package must drop significantly in elevation.  The first hole in the wall, through the surface the unit is hung on must be centered a good inch to two inches below where this package is coming off the back of the unit.  Right from the start this drain line must fall down and away for the condensate collection tray.  The second hole, the hole in the exterior siding of this same wall should be a good 4" to 6" below the first hole.  Then the tubing package must extend straight downwards for a good 3+ feet before turning in any other direction.  That drain line must continually slope downward from the time it leaves the collection tray until the time it drains on the ground. At no time can this drain line run horizontal or slope up even for a short distance.