Glossary
Addresses this service will not probe
Some addresses cannot be probed because of unsupported input, a missing SMTP extension or an unsafe connection target. The result depends on which restriction applies.
An unsupported address literal
The illustrative address user@[192.0.2.1] names an IP address directly. RFC 5321 permits address literals, but Emvero’s registration parser refuses them. Verification stops with DOMAIN_LITERAL_NOT_ALLOWED, undeliverable and reject.
This result reflects Emvero’s input requirements and cannot establish whether the host receives mail. Ask for an address with a domain name. The email syntax explanation distinguishes supported input from the broader SMTP syntax.
A required SMTP extension is missing
A non-ASCII local part requires SMTPUTF8. If the destination does not advertise that extension, the client must not submit the internationalised address, under RFC 6531.
SMTPUTF8_NOT_SUPPORTED records this restriction. Emvero returns unknown with allow_with_email_confirmation because it obtained no mailbox answer. Another attempt against the same server cannot resolve the restriction unless the server’s capabilities change.
A mail route leads to an unsafe target
A mail-server hostname may resolve to a private, loopback or link-local address. Examples include the ranges 10.0.0.0/8 and 127.0.0.0/8. Emvero filters unsafe destinations rather than contacting them for recipient evidence. Repeating a probe cannot bypass that filter.
The vocabulary includes MX_TARGET_BLOCKED for a blocked target, but the code is not guaranteed for every filtered route. If no usable destination remains, the current policy can return MX_UNRESOLVABLE with an unknown result. The MX record explanation separates routing evidence from a mailbox answer.
Responding to the specific restriction
Retain the reason with the address. Request a supported address for an input restriction. Involve the domain administrator when a missing extension or unusable route needs investigation. For an inconclusive check, consider confirmation, a later check or holding the address aside; unknown versus undeliverable explains these choices. The restriction alone gives no evidence that the person submitting the address is fake.
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.
DOMAIN_LITERAL_ NOT_ ALLOWED - Address literal is unsupported input
SMTPUTF8_NOT_ SUPPORTED - Destination does not offer SMTPUTF8
MX_TARGET_ BLOCKED - Code for a blocked connection target; a filtered route may instead report MX_UNRESOLVABLE