---
title: "Why an email address cannot be verified - Emvero"
description: "See how address literals, missing SMTPUTF8 and blocked mail-server destinations limit email verification, and which result or next step applies."
url: https://emvero.io/glossary/unverifiable-address/
language: en
updated: 2026-10-07
---

All terms

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](https://www.rfc-editor.org/rfc/rfc5321.html#section-4.1.3) 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](https://emvero.io/glossary/email-syntax/) 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](https://www.rfc-editor.org/rfc/rfc6531.html#section-3.2).

`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](https://emvero.io/glossary/mx-record/) 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](https://emvero.io/compare/unknown-kept-apart/) 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

## Related terms

### Email address syntax

Address-format rules checked before network lookups. A valid format does not establish a working domain or mailbox.

### MX record

A DNS record that identifies a domain’s mail servers and their priority. A mail route does not prove that a mailbox exists.

### Codes that describe us, not the address

Service protections, capacity limits or incomplete work can prevent a probe. These codes explain the check, rather than the mailbox.

[All terms](https://emvero.io/glossary/)

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