Hospital mailed an letter containing very sensitive health info. /diagnosis.USPS miss delivered the letter. My neighbor opened it with his other mail that day and realized that it belonged to me. He promptly brought it over. The information was compromised. Did the hospital violate HIPPA?

No it didn't violate HIPAA. How would you have expected it be delivered... personal Courrier? In which case it could still be misdelivered. Information is routinely sent in the mail every day. None of it is addressed to "Resident".

It's HIPAA, by the way.

I have had plenty of mail that wasn't mine delivered to me at home and at work. Certainly never opened any of them.
I may be wrong but I have serious doubts that a letter from a hospital would be opened by mistake.
But even in the unlikely even that it was a mistake, the healthcare entity had no control over or responsibility for the error or divulging of protected information

Your complaint is with your local PO. Do complain. They keep track of everything - misdeliveries, thefts, lost mail, etc, so that they can look for patterns by both neighborhood and who the carrier was that day.

As for suing - so close to zero as to be zero.