Can my server convert the address automatically?

No. RFC 6531 offers no general downgrade for the local part of an address, so the message fails unless every hop supports SMTPUTF8.

Do Gmail and Outlook accept UTF-8 addresses?

Gmail supports SMTPUTF8 for sending and receiving. Support elsewhere varies, so check the destination MX before relying on it.

5.6.7 bounce: non-ASCII address not permitted

5.6.7Non-ASCII addresses not permitted for that sender/recipientPermanent: retrying will not help

Typical wording

553 5.6.7 SMTPUTF8 is required, but was not offered by host mx.example.net

What it means

RFC 6531 (SMTP Extension for Internationalized Email) registered X.6.7 as “non-ASCII addresses not permitted for that sender/recipient”. It fires when a sender or recipient address contains UTF-8 characters, such as josé@example.com or an address in a non-Latin script, and a server on the path does not support the SMTPUTF8 extension.

Postfix, for example, produces 5.6.7 on the sending side when the message needs SMTPUTF8 but the remote server never advertised it. A receiving server can also return 553 5.6.7 when it sees UTF-8 in MAIL FROM or RCPT TO without the SMTPUTF8 parameter.

There is no downgrade path for an address itself: unlike body text, the mailbox name cannot simply be re-encoded. Related registry codes X.6.8 and X.6.9 cover UTF-8 replies and UTF-8 headers. Domains in non-ASCII form are less of a problem, because senders can use their punycode (xn--) form.

Likely causes

A recipient or sender local part (before the @) containing accented or non-Latin characters.
A receiving server that does not announce SMTPUTF8 in its EHLO reply.
A sending client that adds UTF-8 addresses without the SMTPUTF8 parameter on MAIL FROM.
An internationalised domain name written in Unicode by a client that does not convert it to punycode.

How to fix it

Check whether the recipient has an ASCII alias, and use that address instead.
If only the domain is non-ASCII, send to its punycode form (for example `user@xn--bcher-kva.example`).
Make sure your own MTA supports SMTPUTF8 (Postfix 3.0 and later with `smtputf8_enable = yes`).
If you are the recipient’s admin and want to receive such mail, enable SMTPUTF8 on every inbound hop including filters.

Checks that apply

Your domain or sending IP (fills in the checks below)

SMTP Diagnostics Shows whether the recipient’s MX advertises SMTPUTF8, which decides whether a UTF-8 address can be delivered there at all.
MX Lookup Lists every MX host for the domain, since each one, including backups, needs SMTPUTF8 for reliable delivery.

FAQ

Can my server convert the address automatically?: No. RFC 6531 offers no general downgrade for the local part of an address, so the message fails unless every hop supports SMTPUTF8.
Do Gmail and Outlook accept UTF-8 addresses?: Gmail supports SMTPUTF8 for sending and receiving. Support elsewhere varies, so check the destination MX before relying on it.

All 191 bounce codes