Files with this naming convention often originate from large-scale breaches. If you are checking to see if your own data is included, it is much safer to use established tools rather than handling the raw text files:
Do you have of this specific file set that you need help formatting or searching through? 1M Maillaccess_000005.txt
Large lists often contain duplicates or malformed entries. You can use tools like (with the TextFX plugin) or command-line tools to clean them: Files with this naming convention often originate from