Use a unique, strong password that you do not use anywhere else.

These lists are rarely from a direct breach of Microsoft itself. Instead, they are typically compiled from smaller breaches of third-party websites or harvested via infostealer malware that captures saved passwords from infected user devices. Recent Related Security Incidents

Verify your "Recent Activity" in your Microsoft account settings to ensure no unauthorized successful logins or syncs from unknown IP addresses.

Microsoft reports that MFA can defeat 99% of password attacks .

While "440k Outlook.com Domain HQ base.txt" is a specific historical file, recent threats targeting Outlook users include:

If you suspect your information may be in such a list, security experts from Microsoft Support and companies like ImmuniWeb recommend: