# Method

## Lookup date

MX and TXT records were looked up on 2026-10-21 through the public resolvers
1.1.1.1, 8.8.8.8 and 9.9.9.9, with `scrapers/dns_detect.py`.

## Counting rules

- The MX host with the lowest preference number decides. A host under
  google.com or googlemail.com counts as Google Workspace. A host under
  protection.outlook.com, protection.office365.us or mx.microsoft counts as
  Microsoft 365.
- When that host belongs to an email filtering service, the domain's v=spf1
  record is read. An include of `_spf.google.com` counts as Google Workspace
  and an include of `spf.protection.outlook.com` as Microsoft 365. When the
  SPF record names neither, the domain counts as behind a gateway with an
  unknown mailbox host.
- A domain with no MX record, or an MX host outside the classified list,
  counts toward the base with no email host.

## Base

The same base as every other Installmap number for the group: domains whose
homepage returned a usable record in the 4 September 2026 crawl. 4,888 Inc
5000 domains, 5,388 Y Combinator, 472 Fortune 500 and 1,000 unicorns.

## Company fields

Employee band, industry and founding year come from the published 2026 Inc
5000 list. YC batch year and status (active, inactive, acquired, public) come
from YC's company directory. Fortune 500 rank comes from the 2026 list.

## Limits

- MX shows where inbound mail is delivered. A company can deliver to one host
  and keep mailboxes elsewhere, and the SPF step only names the mailbox host
  when the company lists it.
- 192 of the 243 Fortune 500 domains behind a gateway have no readable host,
  so the Fortune 500 Google and Microsoft shares are lower bounds.