[__david__]

The problem with your canonicalization idea is that it doesn't work in all cases. Yes, it might work with gmail addresses, but there's no way that you can assume that "[e-mail]" and "[e-mail]" are actually the same mailbox. If you so assume that, then you've just broken you websites for users where that isn't the case.

[fvargas]

Why would you broadly assume an incorrect rule? Email address parsers correctly implement canonicalization rules that consider the domain e.g. gmail. It doesn't require any extra work as a developer and the logic is hidden behind the abstraction. But certainly, you shouldn't go implementing arbitrary rules that aren't reasonably applicable.