Can someone be charged with trespassing if they've lived at that address for 8 months and receives mail at the address?

No, they can't be charged with tresspassing, they are now a resident ( getting mail does not make a difference in most places) They will have to be evicted

Unless you have a specific order against them being at the property then it is not trespassing.

Trespassing occurs when someone enters property without permission. If they have established residency they aren't trespassers.