Glossary
Email address syntax
Email syntax defines the structure of an address: its local part, the @ separator and its domain. A format check looks at that structure without contacting DNS or a mail server.
The local part and domain
In the illustrative address alex+orders@example.com, alex+orders is the local part and example.com is the domain. A parser checks their structure before looking up DNS or connecting to a mail server.
In an unquoted local part, dots can separate groups of characters but cannot appear first, last or consecutively. Thus alex..orders@example.com fails the format check. A plus sign is allowed. Preserve it and the tag when submitting the address so you do not change the recipient. The plus-address guide explains these tags.
Which input Emvero accepts
RFC 5321 permits more forms than ordinary unquoted addresses. Emvero’s registration parser accepts a narrower set: it refuses quoted local parts, address literals such as user@[192.0.2.1], display names and address lists. A form permitted by SMTP may therefore be unsupported input.
INVALID_SYNTAX, INVALID_LOCAL_PART and INVALID_DOMAIN identify a format failure and the part involved. ADDRESS_TOO_LONG identifies a length failure. Emvero’s limits are 64 octets for the local part and 254 for the complete address after domain conversion to ASCII. An octet is a byte, and a Unicode character may occupy several bytes. Receiving servers may apply different limits.
Internationalised addresses
A non-ASCII local part requires SMTPUTF8, as defined in RFC 6531. An internationalised domain can instead be converted to ASCII; it does not, by itself, make the local part require that extension.
Emvero currently accepts internationalised local parts and records smtp_utf8_required. The vocabulary includes SMTPUTF8_REQUIRED for a policy that disallows such input, but this is not a code returned for every supported Unicode address. The unverifiable address explanation covers a destination that lacks the extension.
What passing the check means
A syntax failure can end verification with undeliverable and reject. Passing confirms only a supported format. The domain, mail route and recipient need further evidence, as explained in syntax versus SMTP verification. Keep the original local part intact through those checks.
Related reason codes
The API uses these identifiers to explain a check. The notes identify values that the current verification policy or probe does not return.
INVALID_SYNTAX - Address is not well formed
INVALID_LOCAL_ PART - Local part is not valid
INVALID_DOMAIN - Domain is not valid
ADDRESS_TOO_ LONG - Address exceeds the permitted length
SMTPUTF8_REQUIRED - Defined code for disallowed internationalised input; current policy permits that input