Immediately change passwords for your email and any account that shared that password.

Using the same email/password combinations to try and break into other high-value accounts like Amazon, Netflix, or banking portals [1, 2].

Taking data from a leaked site (e.g., a gaming forum) and "checking" it against mail providers.

These lists are rarely the result of a direct hack on a mail provider like Gmail or Outlook. Instead, they are usually compiled through:

In the context of cybersecurity and data leaks, (or "hits") signifies that a third party has successfully logged into these accounts using automated tools. These lists are frequently traded or shared on dark web forums and Telegram channels for several malicious purposes:

Harvesting personal information, tax documents, or private communications found within the inbox [2, 5]. How These Lists Are Created

Use services like Have I Been Pwned to see if your email has appeared in recent leaks.