What to run
Open Email Validator. It stays in the browser. It checks syntax only and does not prove the mailbox exists.
Open DNS Lookup for MX and TXT. DNS Lookup is DNS-over-HTTPS from the browser and does not follow CNAMEs.
How to read the result
A syntax fail on Email Validator stops the story: the address is malformed.
MX missing or pointing at . means no mail host.
TXT SPF v=spf1 -all with no includes means the domain publishes no permitted senders.
What to open next
Query A and AAAA on the MX target with DNS Lookup.
This chain cannot see mailbox quotas or spam folders.