You could ask the mailman to be more gentle.
Sorry I couldn't resist. I would use limit mates in this case, not perfect but it would achieve the desired result, I think. Considering this is a Mailbox I assume the door is hinged so you can figure the max/min angle of opening and use a limit angle mate.
You are asking for a flexible assembly or rigid assembly?
If you are using flexible assembly, limit the mates with angular measurements(you can find in advanced mates) and check the motion of the assembly, if not, keep the location of door as per your requirement and check the interference(you can see it in the evaluate tab)
Naga Ranjithkumar A
Use a "Limit mate" (Go to mate...Advanced Mates...Angle...notice the option to enter a minimum and maximum angle)
Open/close the door using the "Move component" command with the "Collision Detection" and "Physical Dynamics" options checked.