Cold outreach advice usually talks about "the spam filter" as if it were one thing. It is two things. As of the 2026-07-05 snapshot of our daily Tranco top-1M scan, 21.75% of domains with an MX record route mail to Google Workspace and 16.68% to Microsoft 365 — 38.4% combined, more than any other category including self-hosted (22.53%). If your ICP is B2B, the odds that your next email is judged by one of these two stacks are even higher than the raw domain share suggests.
The two filters are built on different philosophies, fail in different ways, and reward different sending behaviour. Here is what a decade of DNS data says about where each is heading, and a concrete playbook for each side of the duopoly.
The duopoly in one chart
In 2016 Google Workspace already held 15.4% while Microsoft 365 sat at 4.46%. Google grew steadily to 20.11% by 2022 and 21.75% today. But the story of the last four years is Microsoft's: from 12.37% in 2022 to 16.68% now — +4.31 percentage points against Google's +1.64, a 2.6× faster climb. Enterprise migrations, Teams bundling and E3/E5 economics are pulling the market Microsoft's way; our deep dive on the enterprise pull unpacks the drivers. For an SDR the takeaway is simpler: the share of your list behind Exchange Online Protection grows every quarter, and that is the harder filter for cold traffic.
Two filters, two philosophies
| Google Workspace (Gmail) | Microsoft 365 (EOP) | |
|---|---|---|
| Primary signal | Recipient engagement with your domain: opens, replies, spam reports | Sender IP and domain reputation, tenant-level allow/block lists, SCL score |
| Failure mode | Silent categorisation: Promotions tab or Spam, per-user | Org-wide: one admin block or transport rule suppresses you for the whole tenant |
| Recovery | Gradual — engagement from a few recipients lifts domain reputation | Slow and opaque — tenant blocks rarely expire on their own |
| Content sensitivity | Moderate; heavy HTML and link trackers push toward Promotions | Higher; suspicious-looking URLs and spoof-like headers raise the SCL sharply |
| Feedback available | Postmaster Tools (aggregate domain reputation) | SNDS for IPs; per-message X-Forefront headers if you can see a delivered copy |
The Google playbook
- Engineer early engagement. Gmail's model updates on how recipients treat your mail. Front-load your best-fit, most-likely-to-reply prospects at the start of a new domain's life; a few replies compound into reputation.
- Write like a person. Plain formatting, one link at most, no open-tracking pixel on the first touch. Template-shaped HTML is the fastest route to Promotions.
- Watch aggregate signals. Domain reputation in Postmaster Tools reacts within days. If it dips, cut volume first and ask questions later.
The Microsoft playbook
- Respect the tenant. Reputation damage at one company is company-wide. Cap sends per organisation (not per contact) and stop a sequence at the first spam-foldered reply or hard bounce from that tenant.
- Fix authentication to the letter. EOP is unforgiving about SPF/DKIM/DMARC alignment; a misaligned tool in your stack costs you more here than at Gmail. If your prospect also enforces DMARC, read what p=reject changes for outreach.
- Discount your click data. SafeLinks scanning produces machine clicks. Judge Microsoft-side sequences by replies, not opens or clicks.
Operationalising the split
The mechanics are cheap: resolve MX for every domain on your list (see our one-command MX guide), bucket contacts into Google / Microsoft / other, and run separate sequences with separate metrics. Benchmarks that mix the two are meaningless: a 4% reply rate blended may be 7% at Google and 1% at Microsoft — one working sequence and one broken one, invisible in the average.
Provider shares shift every month, and the Microsoft climb shows no sign of flattening. The daily email infrastructure report tracks both curves daily across the full top-1M.
Finally, remember that everything above describes the filter, not your standing with it. The only way to know whether your domain currently inboxes at Gmail and Microsoft 365 is to test: send to seed mailboxes at both providers before each campaign and check the folder, not just the delivery receipt. Placement diverges between the two more often than most senders expect.