$ man mx-record

MX Record

Mail exchange record. DNS setting that tells you which email provider a domain uses (Google, Microsoft, custom).

Not all email providers are equal. I can deliver reliably to Google Workspace from my Instantly setup. I can't deliver reliably to Microsoft 365 or custom Exchange servers. If I try, the emails bounce or land in spam. MX records let me route contacts to the right platform before I send. It's a filter, not a guess.

In Clay, I use the HTTP API to check MX records on every contact's email domain. If the result contains "google.com," the contact routes to Instantly. If it contains "outlook" or "protection.outlook" or anything non-Google, the contact routes to HeyReach. This prevents wasted sends and preserves domain reputation. MX classification is Play 14 in my series — one of the highest ROI filters in the whole workflow.