Files like these are often distributed or sold on dark web forums and Telegram channels.
The "300-K" suggests the file contains approximately 300,000 unique records .
Combolists and ULP Files on the Dark Web: A Secondary ... - Group-IB 300-K HQ-Mail-Access .txt
Attackers use these lists to gain unauthorized access to personal or corporate email accounts.
If you are concerned that your data may be included in such a list: Files like these are often distributed or sold
Automated software systematically tests these leaked pairs against various websites to find matches. Recommended actions
The primary danger is that a password leaked from one service might work on another (e.g., banking or social media) if the user reuses credentials. - Group-IB Attackers use these lists to gain
"HQ-Mail-Access" is common slang in hacker forums for "High Quality" mail access. This indicates the credentials (usually in email:password format) are claimed to be valid or verified for logging directly into email accounts.