What is ESP segmentation?
ESP segmentation splits a recipient list by the mailbox provider that receives each address, such as Google, Microsoft or Yahoo. The provider is what filters the mail, and each one runs its own filtering, so a sender can watch, pace and plan each group separately. Consumer domains reveal their provider on sight; a business domain's provider shows in its MX records.
Reading the result
The table groups the addresses by provider, with each group's share of the list and whether it came from the consumer domain list or an MX lookup. Business domains stay "not looked up" until you ask for the lookup, which sends only the domain names. The inbox table then multiplies each group by the emails per recipient, spreads them over your sending days and divides by what one inbox sends a day.
What it usually shows
Business domains on Google or Microsoft
A company address looks like neither Gmail nor Outlook, yet many companies host their mail at Google Workspace or Microsoft 365. Only the MX lookup shows it, which is why the business share is kept apart until you run it.
A security gateway in front of the mailbox
Some MX records point at a filtering service such as Mimecast or Proofpoint. The lookup then names the gateway: the mailbox provider behind it is not published in DNS.
Domains that take no mail
A domain that does not exist, or that publishes a null MX ("0 ." under RFC 7505), accepts no mail, so the tool leaves it out of the sizing. A domain with no MX record at all is different: RFC 5321 has senders fall back to its address record, so it stays in.
Rounding per group
Each group is rounded up to whole inboxes on its own, so the sum across groups can be higher than one shared pool would need. The tool shows both figures.