As I was bloging before here, the Lotus Notes Client does not support IDN, which means a conversion of the address a user may type in is necessary but currently not available. I was thinking about the problem and found that part of the problem may be fixed in the mail template for the Lotus Notes Client.
You could change all fields displaying mail addresses and all fields used to type in mail addresses to decode to punycode or encode from punycode. You may want to use the existing ICU4J JAVA library to do so.
Although this is a good aproach, problem continues as a user may want to add the senders email address to his address book, so we need to customize the personal address book too.

I found that customization is a first aproach but will result in several new problems. I see problems with 'mail rules' as they are beeing executed on the server, IBM Domino Web Access (DWA/iNotes) as I do not see a possibility to customize it and several other minor issues.

As a result, I would not recommend a customer to try to get IDN support by customizing the templates as there are too many negative aspects:

• You are loosing support from IBM for all problems regarding the customized templates.
• You have to repeat your customization if you are upgrading your Lotus Notes Clients.
• You will not be able to get it work everywhere in Lotus Notes.

So, what has to be done?
IBM is a company providing software that meets their customers business needs - they just have to know about their customers needs. As a customer, contact IBM Lotus Support and ask for IDN support. A PMR has to be opened and an SPR number will be presented to you. This SPR number refers to a document describing the missing feature. Ask for a fix for this SPR, so you will be added to a list of customers, waiting for an implementation. And finally, post a comment here and let the community know what you did.