What Is Mailbox Provider (MBP)?
A mailbox provider (MBP) is a company that operates email inbox services for end users — such as Gmail, Outlook/Microsoft 365, Yahoo, and Apple iCloud Mail. Each MBP runs its own filtering, reputation, and authentication rules, so deliverability must be earned and measured provider by provider.
MBPs are the gatekeepers of the inbox. Every one maintains its own private reputation system and filtering logic, which is why the same email can inbox on Gmail and land in spam on Outlook. There is no universal "deliverability" — only per-provider outcomes.
A few providers dominate most audiences: Gmail and Microsoft (Outlook/Office 365) together account for the majority of business and consumer email. Understanding each one’s expectations — Gmail’s emphasis on engagement, Microsoft’s stricter reputation gating — is central to deliverability.
Because rules differ, best practice is to monitor placement per MBP and match your warmup network’s provider mix to your real audience, so the reputation you build applies where it matters.
MailPilot breaks placement down per mailbox provider and balances its warmup network across Gmail, Outlook, and others, so you build reputation with the providers your prospects actually use.
See the provider network →