As suggested on reddit, they all might belong to the same spammer. I don't know about hotmail account creation, but if it's protected by captchas only spammers have no problems making the accounts. Some captchas can be decoded in code with high success relatively quickly, others can be fed to mturk style services.

Google has phone verification now, I guess that's harder to overcome.

Not a smart thing to do (to log into those accounts).

If you found a bunch of house keys, each one labled with the address of the house, would you go to each house and open each door? Stepping inside is not necessary.

That of itself, might not break any laws (without prior warning, or intent to cause harm, it might not be trespassing), but computer tresspass laws only require unauthorized access.

Easiest thing would be to send the list to Hotmail

Is there a reason why the list contains Hotmail accounts only? Does this mean Hotmail users are easier to hack into?

See. I'm torn. I know it's illegal, but if you present the information about the list to a company (without logging in to the accounts), and they don't take action, what do you do?

I suppose you could email all of the people on the list, but how effective would that be?

From the look of it, this need not have to be hacked accounts. Normally spammers create thousands of accounts on free email services and the bots are used to auto login and send spams out. Mails sent from hotmail, msn etc has better chances of reaching your inbox than that sent from custom domains which could easily end up in spam folders.

The spammers favorites are hotmail and msn as they are easy to create. Gmail has phone verification and other added stuffs that makes it difficult for bots to create accounts.

The server that redditer accessed could be the spammer's server where he stored the user name / passwd in plaintext format for the bots.

I say the chinese government is building a massive list of email password pairs that they can use down the road.