Is the US Post Office allowed to remove mail from a mailbox because the mailbox is full?

Yes, they just did it to me today and I am furious. I am getting ready to sell my home and am living in another town, but occasionally still stay there. Today I went to get my mail, which I do every couple of weeks with no problem. Today there is a card saying “Vacant. Do not Deliver Mail Here”. What??? They say they removed all my mail, because it was too full to fit anything else. So? Don’t fit anything else! But, don’t take my mail that’s in my box and belongs to ME! It was already in there. Why take it? Now it’s been sent back, don’t know where everything was from. How dare they?