What this checks, and what it deliberately does not#
This validates syntax only: whether the address is shaped like a real email address according to the practical rules that matter — one "@", a local part and domain within length limits, no consecutive dots, a domain with a real-looking top-level domain. It does not check whether the domain actually exists, has mail servers configured, or whether the specific mailbox is active — those require a DNS lookup and often an SMTP handshake, which a client-side tool with no backend cannot perform. A syntactically perfect address can still bounce; this tool answers "is this shaped correctly," not "will mail actually arrive."
Why every failure gets a specific reason#
A bare "invalid" tells you something is wrong but not what, which matters when cleaning up a list of hundreds of addresses collected over years from different sources — a form, a CSV import, a manual paste from an old spreadsheet. Knowing that one address failed because of consecutive dots and another because of a missing top-level domain is the difference between a five-second fix and re-deriving the problem by eye for every flagged row.
Common reasons a real address gets flagged#
A stray space from a copy-paste is the single most common cause — invisible in a spreadsheet cell, immediately fatal to the syntax. A trailing or double dot in the local part ([email protected]) is a frequent typo. A missing top-level domain (user@localhost, user@company) is valid inside a private network but not a real internet email address. All three are common enough that a bulk list pulled from years of manual entry usually has at least one.