[voidlogic]

Assuming you are trying to prevent people from creating multiple accounts on your service... keep in mind they could create e-mail forwarders at other domains, this just stops a very easy method.

Also, more importantly, this would have to only apply to a set a whitelisted domains, plenty of non-gmail systems would treat these variations as distinct addresses. So without a set of domains to apply this logic too, you could end up stopping users from registering with their own normal/actual address.