I disagree. While n@ai might be a technically valid email, its such a extreme edge case ( maybe 1 out of 1,000,000 people have a email like this) that its worth denying that person registration to keep the likely thousands of erroneous emails from being entered incorrectly and the time that goes into correcting them. Same thing goes for addresses like "<>;@\'`{}|.a"@παράδειγμα.δοκιμή

Honestly, if you decide to use a email like n@ai you already know what to expect. Most services wont let you sign up, And even if they do most will likely incur errors in the application when you attempt to do things.

In reality, while it may be 'in spec' to use such a email, we can all hope that edge cases that allow it are changed and the legacy 'rules' that allowed it in the first place phased out completely.

So, in practice in the 'real world'- n@ai is not a valid email address and never will be. If I create a web application you can bet your bottom dollar I wont allow it and I will create less work for myself by doing so.