About Validmailbox

Why this site exists

Deliverability advice online tends to arrive in two forms: a list of tips with no way to check whether they apply to you, or a paid platform that wants your list before it will tell you anything. Both are frustrating when the question in front of you is small and specific. Does this domain accept mail. How many days until this mailbox is warm. Is this subject line too loud. Is my SPF record over the lookup limit. Validmailbox answers those four questions and nothing else.

How the tools are built

Three of the four tools run entirely in your browser. The address inspector parses syntax locally, matches against embedded role and disposable lists locally, and computes typo suggestions locally. The warmup calculator and the spam checker never contact a server at all. The record generator assembles strings from what you type. The single exception is the MX lookup, which cannot run in a browser without help, because browsers have no DNS API and no access to port 25.

The MX lookup

For that one check, the domain part of the address is sent to Google's public DNS-over-HTTPS endpoint, which returns mail exchanger records as JSON over HTTPS. Only the domain goes; the local part of the address stays in the page. If your network blocks that endpoint, the request falls back to a resolver on this site, and the result tells you which source answered.

What we deliberately do not do

• No SMTP probing. We do not open a connection to a recipient's mail server to ask whether a mailbox exists.
• No list upload. There is no bulk mode, no CSV import, and no account.
• No storage of what you type. Addresses, copy, keys and domains are not logged or retained.
• No inbox placement claims. No public tool can see the signals that decide placement.

SMTP verification is the omission people ask about most. It is left out for two reasons. It is unreliable, because most large providers accept every recipient during the SMTP conversation and bounce afterwards, so a confident answer is often wrong. And it is rude: repeated probing from one source gets that source blocked, which degrades the check for everyone using it.

Where the data comes from

The syntax rules follow RFC 5322 for the address grammar and RFC 5321 for length limits. The authentication guidance follows RFC 7208 for SPF, RFC 6376 for DKIM and RFC 7489 for DMARC. The disposable domain list and the spam phrase dictionary are maintained by hand from public lists and from patterns that show up repeatedly in filtered mail. Both are bundled into the page rather than queried, which is what keeps the tools fast and private.

Accuracy and corrections

These lists go stale. New throwaway providers appear weekly, and filter behaviour shifts. If a check gives you a result you can show is wrong, send it over and it gets fixed. Corrections with a reproducible example are the most useful thing anyone sends us.

Who runs it

Validmailbox is maintained by a small team that works on email infrastructure and outreach systems. The site is funded by advertising, which is why you see ad placements on these pages. It does not sell data, run affiliate placements inside tool results, or change what a tool reports based on who is paying. Questions go to the contact page.