Verify one email address

Checks a single address and returns one of four outcomes with a plain-English reason. Note that a confirmed mailbox is only one of them — on a mail server that accepts every address the outcome is risky, which is deliverable-but-unconfirmable rather than bad. Answers we cannot confirm are always free.

Required scope: verification:write.

Accepts an optional Idempotency-Key header so a retry never spends a second credit.

Results are one of valid, invalid, risky or unknown. risky means the domain accepts every address, so delivery cannot be confirmed. unknown means no definitive answer — never charged.