In general angle mates are pretty "flaky". And going to an angle of 0 is a pretty big no no. Not that it is against the rules, it is just that Solidworks gets confused.
My advice is to create a plane in the body portion of the mailbox that would be coincident with the door in open position. Then make as many planes as you want to show different positions of the door.
Then at the assembly level, make the t door coincident to each plane separately in different configurations.
I don't have time to open the model right now, but you might try angles of something like 0.01 and 179.5 instead of 0 and 180, although I'll certainly agree with Dan about angle mates being flaky. Fortunately I rarely need to use them in my work.