Robust email address syntax and deliverability validation library (Python 3)

 This library validates that a string is of the form x@y.com. This is
 the sort of validation you would want for an email-based 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!).