$ man provider-split

Why You Split Google + Microsoft

The practice of maintaining sending infrastructure on both Google Workspace and Microsoft 365 instead of picking one. You split because your contacts use both, and provider-to-provider matching improves deliverability.

If 60% of your contacts use Google and 40% use Microsoft, sending everything from Google means 40% of your emails are crossing provider lines. Cross-provider sends have lower deliverability — Microsoft doesn't trust Google's sending servers as much as its own. By splitting your infrastructure, you match sender to recipient at the provider level. Google sends to Google. Microsoft sends to Microsoft.

When I set up infrastructure through Inbox Kit, I specify a provider split based on the ICP's likely inbox distribution. If the partner targets SMBs (mostly Google), I go 70/30 Google/Microsoft. If they target enterprise (mostly Microsoft), I flip it. The MX record check in Clay determines which contact goes to which provider. The split isn't always 50/50 — it follows the data.