We had this through the wall A/C installed in May with new sleeve kit and separately purchased protective rear grille. Started smelling very bad on the inside (room side) several weeks ago. It appears to be at the correct angle for drainage, and the grille has drainage holes, yet we never noticed any dripping outside, even on the hottest days. What is likely the cause? What can we do?

Well! I think that your A/C is not working properly may be because of high temperature or improper installation. For reducing your room temperature you can try this out which has proved beneficial to one of my friends also and now she is not a regular user of A/C as the new style doors and windows cools her room and ultimately saves electricity.