Files like these are often distributed or sold on dark web forums and Telegram channels.
"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. 300-K HQ-Mail-Access .txt
These are standard plain-text files designed to be fed into automated tools for credential stuffing . Security risks and implications Files like these are often distributed or sold