Mail routing between Domino and domains with Internationalized Domain Names (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@bü To support IDNs, Domino SMTP servers that route inbound and outbound mail require Domino 12.0.1.

Domino mail users can address mail to the IDN user using either the international domain name (martha@bü or the Punycode ASCII compatible encoding for the domain name (

When a Domino user sends a message to martha@bü, 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 in the current release:
  • International Email Addresses (EAI) are not yet supported. Sending mail to and from addresses such as jönas@bü, 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.