Also, email addresses ARE case sensitive. While the domain name is not case sensitive (but of course, domain names only contain ASCII characters!), the local part of email addresses ARE case sensitive, and <John@foo.com> can be very different from <john@foo.com>!
211 of the top 500 domains (https://moz.com/top500) contain the letter 'i' within them. It may be worth checking out if they are also susceptible to the Turkish dotless 'i' problem.
Their "awesome Unicode" guide is worth looking into as well, for those who find this article interesting.