A bad filter reaches thousands of inboxes before anyone notices. List Gate reads every row of your CSV before it goes into a sending tool and gives one answer: go, or hold. The file is read in your browser and never uploaded.
The mailbox half of cold-email-list-gate, run on every row, plus the ten-row sample the skill asks a person to read. The verdict uses the skill's rule: two wrong in ten is a pattern, so hold and fix the source.
Nowhere. Your browser reads the file from your own disk and the script in this page checks it. The rows are not sent to us or to anyone else, which matters because a lead list is other people's personal data. You can read the script at /list/list-core.js.
No. That takes a verification service that contacts each mail server. This check finds what is wrong on the page before you pay for verification, and the result says so.
You can download the clean rows, but the skill's advice is to fix the filter or the provider that built the list. When one row in five is wrong, the rows you cannot see are wrong in the same way.
Fit is half the gate. Without your titles it can only check the addresses, and the verdict is marked as mailboxes only. Company and size fit still need you to read the sample.
That depends on your country and on who you write to, and this tool does not check it. Find out before you send.