About
Five tools for the two questions email keeps raising: is this address real, and why did the message not arrive. Four of them only inspect, so nothing is delivered and nobody is contacted. The Outlook reader is the exception and needs a token you already hold.
Debugging delivery in the right order
MX first, because it says where mail for the domain is supposed to go and it frequently reveals a leftover record from an old provider. The SMTP test next, against the host the MX record named, to confirm something answers there. Headers last, once you have a failed or suspicious message in hand. Reversing that order means reading a Received chain to diagnose a problem that was a stale DNS record.
What stays in your browser
Header analysis is local, so a message you would rather not hand to a website can still be examined. The address check, MX lookup and SMTP test all run from our server, because they need to reach the internet from somewhere other than your machine. The Outlook reader passes your token to Microsoft for that one request and keeps nothing.
The one tool with a credential
Reading an Outlook mailbox requires a refresh token, and that changes the terms. Use it only for mailboxes you are entitled to read, clear the field when you are done, and be aware that a token is a standing permission rather than a one time password. If you do not already hold one, nothing here will produce it for you.
FAQ
Will any of these send a message?
No. Address checks, MX lookups, SMTP tests and header analysis all stop short of delivering anything. The SMTP test reads the greeting and disconnects.
Can I verify addresses on my own domain?
Not directly. The address checker covers @gmail.com. For another domain, the MX lookup and SMTP test tell you whether its mail server is reachable, which is a different and often more useful answer.
Is header analysis done on your server?
No, it runs in your browser, which is deliberate given that headers carry addresses and internal hostnames.
Related tools
Verify Gmail addresses, test an SMTP server, look up MX records and analyse message headers. None of these tools send mail to anyone.