This library validates that address are of the form x@y.com. This is the sort
of validation you would want for a login form on a website.

Key features:
* Good for validating email addresses used for logins/identity.
* Friendly error messages when validation fails (appropriate to show to end
   users).
* (optionally) Checks deliverability: Does the domain name resolve?
* Supports internationalized domain names and (optionally) internationalized
  local parts.
* Normalizes email addresses (super important for internationalized addresses!).