Mail routing between Domino and domains with IDNs

Domino mail users can now communicate with non-Domino mail users who use internationalized domain names (IDNs) in their email addresses.

For example, a Domino user can send mail to and receive mail from a user with an address such as martha@bücher.com. To support IDNs, Domino SMTP servers that route inbound and outbound mail require a minimum of 12.0.1 for the Domino version.

Domino mail users can address mail to the IDN user using either the international domain name (martha@bücher.com) or the Punycode ASCII compatible encoding for the domain name (martha@xn--bcher-kva.com).

When a Domino user sends a message to martha@bücher.com, the router converts the recipient's domain to Punycode and finds the appropriate MX host in DNS to transfer the message over SMTP.

When a a non-Domino IDN user sends a message, the Domino SMTP server converts any UTF-8 encoded domains in the MIME address header fields to Punycode, if necessary. MIME headers related to mail threading (conversations), are also converted to Punycode if they were encoded with UTF-8. Note that many IDN senders will send Punycode headers directly in the MIME messages.

For more information, see the wikipedia entries Internationalized domain name and Punycode.

Note: The following features are not supported:
  • International Email Addresses (EAI) are not supported. Sending mail to and from addresses such as jönas@bücher.com, where there is an international character in the local part (left of the @), will not route properly and result in delivery failures.
  • Domino cannot host an international domain name. Person documents in the Domino directory support only internet addresses with ASCII characters.