$ man mx-record-lookup

MX Record Lookups

Querying a domain's DNS to find its mail exchange (MX) records. The MX record tells you which email servers handle mail for that domain — Google, Microsoft, Mimecast, Barracuda, custom Exchange, etc.

You can't route contacts effectively without knowing their inbox provider. If you guess wrong, your emails hit spam or bounce. MX record lookups remove the guessing. They tell you exactly which provider a company uses for email. That one data point determines your entire routing logic — which platform sends the email, which infrastructure it sends from, and whether you email at all.

In Clay, I run an MX record lookup on every contact's email domain using the HTTP API integration. The API returns the domain's MX records (e.g., 'aspmx.l.google.com' for Google, 'mail.protection.outlook.com' for Microsoft). I parse the result with a formula column that classifies the provider. This classification feeds the routing decision downstream. MX lookups are one of the cheapest, highest-ROI enrichment steps in any campaign.