Google and Microsoft run 25.5% of mail-operating domains with a named provider

The domains no named provider serves

23.1M mail-operating domains, 13.2% of all of them, have a primary MX the catalog does not attribute to a named provider. They are left out of the ranking above because they are not one provider or a few: they are what remains once every cataloged provider has been matched, and a single bar for them would read as the largest provider on the list.

Most of them are not on a provider at all: 72.86% have a primary MX named under the domain itself, such as mail.example.com, and 9.21% have a primary MX that cannot take mail. The 17.62% on a third-party MX domain are a long tail rather than a gap in the catalog: they are spread across 431,212 MX domains, and the 1,000 largest of those serve 2.92% of all the domains with no named provider.

Sized by how many domains each of those MX domains serves, 45.2% of the third-party share sits behind ones serving 100 or more, 34.21% behind ones serving 10 to 99, and 20.59% behind ones serving fewer than ten. The largest serves 7,634 domains, so there is no missing provider of any size to find: cataloguing further names the tail one small operator at a time.

They publish SPF at 77.7% against 79.3% for the named providers together, DMARC at 37.5% against 39.4%, and enforce DMARC at 14.9% against 13.7%. On authentication the remainder is not a less protected corner of mail: its rates sit within a few points of the named providers'.