what happens if a neighbor reports you to cdc just cause you went outside?

The CDC doesn't take complaints about people going outside.  

You are fine. Even with a stay at home order you are allowed to do essential things. Getting your mail is essential. 
Most places are allowing walks etc as long as you keep the 6ft distance.   

In most places it's you get fined.
It's quite a hefty cost, not one that any poor or jobless person could afford.

In some places, if you're caught with symptoms they'll force you into two weeks supervised isolation.

Nothing. 

No.

No one really knows. 

Don't be whiny.  If you have to go out cover your face and wear gloves and stay 10 feet away from every and anyone.  You know what the rules are in your state and city....follow them. 

The innocence of this question made me smile. 

First, stop talking to your crazy neighbors. Really. Dollars to doughnuts, this is the same one that was freaking out saying your other neighbors are doing hard drugs just because they might smoke pot and are going to burn the place down. And if it's not the same person, you have some seriously nutty neighbors with no grip on reality. Take anything they say with a pound of salt. 

Secondly, the CDC has some serious things going on right now. They don't care what you're doing. Promise. Sure, they want you to follow their recommendations, but they aren't enforcing things. That's not their department and they have WAY bigger fish to fry.

Thirdly, if it was illegal for you to go get your mail, why would the federal government still be delivering mail?

Fourthly, if it was illegal for you to go outside, it would, in all likelihood, be local law enforcement who handles this claim. You'd need to check with your local authorities to see what rules they've established. I know someone with well-intentioned advice responded here saying there would be fines... that person is not even in the United States. And I can tell you that, even within the U.S., there are HUGE variances in what different states, counties, and cities are doing. For example, here in Arizona, the governor specifically said it was possible that local law enforcement would start issuing fines, BUT IF they ever did, they'd be targeting people gathering in groups of ten or more- they won't go after the individual people. They want to break up whoever is hosting those groups... kind of like how a lot of places used to manage pot. They didn't really care about the single end-user with his joint. They cared about the dude carrying boatloads of it across state lines.  i.e. If people are hosting house parties or restaurants are staying open, local law enforcement would go in and potentially fine the hosts. 

Based on what I could find, Colorado is handling it much the same as AZ is. Fines are possible, but they're not leveraging them now. If it got to the point where things were a concern and people weren't staying home, they'd go after the places that were responsible for group gatherings. You can read news coverage on it here.