Dear Yousri,
A mail address contains of "local part" and "domain" in the format of
local_part@domain.tld
So if you created a domain called مصر.مصر you still need to follow the RFC-822 of a mail address. I guess this would then look like local_part@مصر.مصر
I know you write right to left but I don't know if the RFC supports right to left at all. So this is the most likely issue with the ldap you facing as you try to create a user with a domain of على rather then مصر.مص.
The second part, IDN (defined by RFCs 3490, 3491, and 3492) only covers the domain part of email address.
IDN itself does not have anything to do with local-part. RFC 2822 (section 3.4.1) allows only ASCII characters in the local-part.
There are experimental RFCs 5335/5336 that describe internationalized email addresses. Those have not been adapted yet in the industry.
The best way to avoid this would be not to use international characters in mail addresses and domains. I know it does not really help you in what you are trying to do but I also know that not all mail system in the world will support international characters which means even if Zimbra would support it this user will not be able to send mails to certain users or receive mails from certain users depending on what mail system they are using.
Do Zimbra fully support IDN or are there known exceptions? E.g. does it even work with languages like Greek, Cyrillic and Arabic?
Yes - as far as its QAed. It works fully in terms of functionality (mail transporting, UI, mail filter, provisioning, ...). But I am sure QA didn't test in all possible languages - they should just work. Please file bugs if any issue is found.
I hope this helps a bit
Best Regards
Anders Rydell
Bugzilla
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IDN and zimbra 
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