What to run
Open Email Validator. It stays in the browser.
How to read the result
Valid means the shape is plausible: local part, @, domain.
It does not mean the mailbox exists, accepts mail, or is free of typos.
name@domain without a dot can still be syntactically legal.
What to open next
Open DNS Lookup for MX on the domain. It is DNS-over-HTTPS from the browser and does not follow CNAMEs.
Open Remove Duplicate Lines before you validate a list. It stays in the browser.