This is the most common source. When a third-party website (like a gaming forum or a small e-commerce site) is hacked, their user database is leaked. If you use the same password for that site as you do for your Hotmail, your credentials end up in a list like this.
A "teaser" provided by a seller to prove their larger database is high quality. 1.2k VALID HOTMAIL.txt
The file "1.2k VALID HOTMAIL.txt" appears to be a text document containing a list of email addresses, specifically those ending with the "@hotmail.com" domain. The "1.2k" in the filename suggests that the file contains around 1,200 valid Hotmail addresses. This is the most common source
If you are concerned your account might be on such a list, take these immediate steps: take these immediate steps: