The emails are partially checked in that we check them for the right syntax, that they're formatted correctly, that they aren't any known honeypots/spamtraps and then that the domain exists and has a server accepting email. The only check we miss out is if the user exists at the domain.

To get fully accurate and guaranteed lists you'd need to run them through a service such as NeverBounce or Debounce - Personally I don't and never have, but it's a personal choice.
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