Email syntax checker

Why syntax is harder than a regex

Most sign-up forms validate email with a regular expression, and most of those regular expressions are wrong in both directions: they reject valid addresses with unusual but legal characters, and they accept strings no mail server would ever deliver to. The actual grammar, spread across RFC 5321 and RFC 5322, allows quoted local parts with spaces, IP-address domains in square brackets and comments in parentheses, almost none of which real providers accept. The practical rule set is narrower: what Gmail, Microsoft and the open-source MTAs will take.

The rules this tool applies

The checker walks the same ordered rules Verifyre's engine uses, and reports each one. The whole address must be 254 characters or fewer. It must contain no spaces or structural characters such as angle brackets, commas or semicolons unless the local part is quoted. There must be exactly one @ separating the local part from the domain. The local part is limited to 64 characters, may use letters, digits and the punctuation RFC 5321 permits, and may not start, end or contain a doubled dot. The domain must contain at least one dot, each label must be 1 to 63 characters of letters, digits and hyphens without leading or trailing hyphens, the whole must fit in 253 characters, and the top-level domain must be non-numeric and must actually exist in the IANA root zone, so user@example.con fails. Internationalised domains are converted to punycode for DNS, and non-ASCII local parts are noted as needing SMTPUTF8 support.

Finally the domain is compared with popular providers to catch near misses: gmai.com, hotmal.com, yahooo.com and mangled endings like gmail.cmo. A valid address with a likely typo is reported as valid but suspicious, with the corrected address one click away.

What syntax cannot tell you

A well-formed address is only the first stage. The mailbox may not exist, the domain may have no mail servers, or it may be a disposable provider. Run a suspicious address through the email checker to ask the mail server directly, and use the typo fixer when you have a whole list to tidy.