Block free email domains on B2B forms: the 4,768-domain list and a free API

Unif API has a free email-check API built from the 4,768-domain list HubSpot uses to block free email providers. Its cookbook explains how to apply the rule to signup forms, lead lists and agents.

You can check an address or submit up to 1,000 addresses per batch. All four email endpoints cost 0 credits per call and work with a zero-balance workspace. There's also an open-source package for offline checks.

The list uses HubSpot's October 10, 2026 snapshot. Matching is exact, with no network or MX lookups, so a business verdict doesn't confirm that a mailbox works. Personal addresses can belong to real buyers, and the list misses some newer providers. Unif suggests routing those signups rather than blocking them.