The API can return the following results which determine deliverability. Both GOOD and RISKY verdicts are safe to send emails to.

GOODThe email address passes our tests and can be safely emailed
INVALIDThe email address' format is incorrect or the remote mail server does not exist
UNKNOWNCould not run all validations in time. You may want to try validating using webhooks.
REJECTEDThe remote mail server rejected accepting emails for this email address
RISKYThe email server employs an anti-spam technique which doesn't allow us to detect deliverability. Emails can still be delivered but there is no guarantee it'll be delivered to the addressed inbox.