Technically, this isn't a bad shot. I think the biggest problem is just the subject. Why did you choose this? This topic gives you so much leeway in choosing a great topic. You basically could take the best shot you can think of... and this is it?
As far as improving the shot that you chose... I think the composition needs a little help. The frame is divided in half by the mailbox and road. It'd work better to have that strong line further away from the center.
It's also bothersome to have those cars right along the top. I would have tried to put them behind the box to avoid the distraction. Doing just a shot of the mailbox in itself might be ok. If you could isolate it better or put a more interesting background behind it and do one of those things where you desaturate everything but the red flag. It just needs more interest to stand out.
Technically, you can take a good shot of your subject. You just need to be more imaginative with your subjects. People need to care about your picture and want to spend more than two seconds looking at it.
Look at other shots that you like and what subjects have been chosen. The trick is to come up with a good subject and an interesting way to show it.
I think the mailbox should fill more of the frame, even though you have followed the "rules." I find myself focusing on the cars because that is where the road leads.
Ahh, the anticipation of that letter or package. I know that one well... but with the mailbox taking up less than half of the photo it seems to take away from the emphasis. Looks good in B&W, though.
A good concept... although I think it may have been stronger had the placement of the car behind the mailbox been a little different. The car seems a little awkwardly-placed as it is here.
The mailbox may have been more interesting if you had angled the parked car out of the picture...More attention on the worn and ashen detail on the box itself?