It's per the RFC standard but it cannot be blindly followed in Gmail due to their signups issue. Yes, they did allow A.B@gmail.com and AB@gmail.com to sign up,

It's per the RFC standard but it cannot be blindly followed in Gmail due to their signups issue. Yes, they did allow A.B@gmail.com and AB@gmail.com to sign up, and each of these is mapped to a different Inbox. And an email that goes to A.B@gmail.com also goes to AB@gmail.com. And yes, this opens Gmail users for attacks, phising amplification and much more!