A bulk MX lookup reads the MX records of every domain on a list and tells you which mail host takes each one's email. Paste addresses above in any format, one per line or a column from a sheet, and the map sorts them by host.
Only the unique domains are sent, to Cloudflare's public DNS resolver. The addresses stay on this page, and nothing is stored.
How to read the result
Each domain is labelled by its first MX host, the one with the lowest preference number. Senders try that host first, as RFC 5321, the SMTP standard, sets out. A host is named only when its own setup docs give that MX value, and anything else is shown as it is.
A security gateway is the result people misread. It shows who scans the mail on the way in, not where the mailbox lives, so the company behind it could be on any host.
A domain with no MX record falls back to its own address record, which is usually a website with no mail service. A null MX says the domain takes no mail at all.
What to do next
Remove the domains that cannot receive mail before the first send: ones that do not exist, ones with a null MX, and most with no MX record. Those bounce, and bounces cost the reputation of the mailbox that sends them.
Give personal addresses on a business list a second look. They usually came from a different source than the work addresses beside them.
Then check the addresses themselves. To start your next list from addresses that already check out, find and verify emails for a whole list in one run.