Microsoft passes Google at about 50 employees in the Inc 5000

Key findings

1. Among Inc 5000 companies with 11 to 50 employees, Google Workspace runs email for 47.9% and Microsoft 365 for 41.2%. One size band up, at 51 to 200 employees, Microsoft leads 49.4% to 42.2%.
2. Microsoft keeps gaining with size, to 63.4% of companies with 1,001 to 5,000 employees, where Google has 22.9%.
3. Company age moves the split even more than size. 70.3% of Inc 5000 companies founded before 2000 are on Microsoft, and 53.7% of those founded since 2015 are on Google. Size still matters inside each age group.
4. Y Combinator companies are on Google 79.1% to 5.2%. Among the ones YC lists as active, Google runs email for 86% to 93% in every batch block.
5. The Fortune 500 goes the other way: 36.2% Microsoft and 4.2% Google, which is 20 companies. 46.6% route their mail through Proofpoint first.

We expected company age to explain the whole split, on the assumption that a company picks its email host in its first months and rarely moves. Age does explain most of it. What surprised us is that within companies of the same age, the bigger ones are still more likely to be on Microsoft.

The flip happens between the 11 to 50 and the 51 to 200 bands

Source: Installmap, October 2026.

The employee bands are the ones the Inc 5000 list publishes, so "about 50" is as close as the data gets. The two bands on either side of the flip are large, 2,022 and 1,469 companies, and the 95% intervals do not overlap: Google is 46% to 50% in the smaller band and 40% to 45% in the bigger one, Microsoft 39% to 43% and 47% to 52%. The 22 companies above 5,000 employees are too few to read.

Larger companies also put a filtering service in front of their mail more often, from 6.9% of the smallest to 26.3% at 1,001 to 5,000 employees. Proofpoint is the most common one in the Inc 5000, on 331 domains (6.8%), ahead of Barracuda on 112 and Mimecast on 97.

Age matters more, but size still matters inside each age group

A company founded in 2018 is more likely to be small today, so we checked whether size only stood in for age. It does not.

In every age group Microsoft gains between 9 and 13 points from the smallest companies to the largest. Between the youngest and the oldest age groups the gap is about 30 points at every size. Among companies founded since 2015, Google still leads at every size, and the two only meet at 201 employees and up.

By founding year alone, Microsoft runs email for 70.3% of the companies founded before 2000, 61.4% of those founded 2000 to 2009, 44.5% of 2010 to 2014, and about 37% of everything since 2015.

Government contractors and IT shops are on Microsoft, agencies are on Google

IT Services is the one that stands out to us. Many IT services firms manage Microsoft 365 for their clients, though we did not check how many of these 388 do, and 72.7% of them run their own mail on it. The full table for the 22 industries with 60 or more companies is in the dataset.

YC is a Google shop in every batch

Y Combinator companies run Google Workspace on 79.1% of 5,388 domains and Microsoft 365 on 5.2%. Unicorns sit between YC and the Inc 5000, at 62.1% Google and 19.9% Microsoft.

The rise across batches is not a trend. YC's directory marks each company active, inactive, acquired or public, and inactive companies often no longer receive mail at their domain: in the 2019 and earlier block, 29% of the 412 inactive companies have a Google or Microsoft host, against 93% of the 788 active ones. Counting active companies only, Google runs email for 89.5% of the 2019 and earlier block, 86.5% of 2020 to 2021, 88.2% of 2022 to 2023, 88.2% of 2024 to 2025 and 92.5% of 2026. Microsoft stays between 3.6% and 6.2% throughout.

The Fortune 500 hides behind Proofpoint

Of the 472 Fortune 500 domains we counted, 171 (36.2%) run Microsoft 365 and 20 (4.2%) run Google Workspace. 220 (46.6%) send their inbound mail through Proofpoint first, and 243 (51.5%) through some filtering service.

For 192 of those 243, the public SPF record does not name the mailbox host behind the filter, so we cannot tell Google from Microsoft. The Fortune 500 shares above are a floor for both.

The 20 Fortune 500 companies we can see on Google Workspace include Alphabet, which would be odd otherwise, and Uber, Netflix, Salesforce, Airbnb, Wayfair, Block, Booking Holdings, Carvana, Motorola Solutions and Whirlpool. The full list is in the dataset.

How we counted

The email host comes from DNS, not from the homepage. On 21 October 2026 we looked up every domain's MX records through public resolvers and matched the host with the lowest preference number: a host under google.com or googlemail.com counts as Google Workspace, protection.outlook.com as Microsoft 365. When that host belongs to a filtering service such as Proofpoint, Mimecast or Barracuda, we read the domain's SPF record and count the mailbox host it includes, _spf.google.com or spf.protection.outlook.com. If the SPF record names neither, the domain counts as behind a gateway with an unknown host.

Each share uses the same base as every other number on Installmap for that group: the domains with a usable homepage record, 4,888 for the Inc 5000, 5,388 for YC, 472 for the Fortune 500 and 1,000 for unicorns. Employee band, industry and founding year come from the 2026 Inc 5000 list. 270 of the 4,888 Inc 5000 domains have no detectable email host: no MX record, a host we do not classify, or a gateway with no readable SPF.

The tables are in the dataset, and every tool has its own page per group, for example Microsoft 365 in the Inc 5000 and Google Workspace in YC.