This might already filter out legitimate users. dig MX io. Also, you don't know what new TLDs ICANN might come up with at some point. I am pretty sure there

This might already filter out legitimate users. dig MX io. Also, you don't know what new TLDs ICANN might come up with at some point. I am pretty sure there are service providers that do and did limit the TLD length and then TLDs like xn--vermgensberatung-pwb came up.