Would it make sense to cache (in memory) dns records that got a match to prevent a large amount of dns lookups for common email providers like gmail.com, hotmail.com etc?

If also caching non matches is fine functools.lru_cache decorator could be added to validate_email_deliverability, otherwise some custom logic could be written to keep a small cache of recent valid domains.

What do you think? I could probably make a PR if you like it?