Is this address real?

Type an address. It gets parsed against RFC 5322 rules in the browser, then the domain's mail servers get asked whether the mailbox actually exists.

1. 1

   Parse, don't regex

   Local part, quoting, dot placement, label lengths, IDN, TLD shape. Length limits per spec: 64 and 255 octets.

2. 2

   Reputation signals

   Disposable providers, role accounts, and near-miss typos of the big mail hosts.

3. 3

   DNS

   MX records, implicit A/AAAA fallback, plus SPF and DMARC as a sanity read on the domain.

4. 4

   SMTP conversation

   EHLO, MAIL FROM, RCPT TO against the lowest-priority MX. No message is ever sent. A random address is probed too, so catch-all servers get flagged instead of faking a pass.