What is a mailbox provider (MBP)?

A mailbox provider (MBP) is a service that hosts email accounts and manages the delivery, storage, and retrieval of messages for end users. Think of them as the harbor masters of email, they control which ships (messages) dock safely in the inbox and which get turned away or sent to quarantine.

Major mailbox providers include Gmail, Yahoo, Microsoft Outlook, and Apple iCloud. Each operates massive filtering infrastructure that evaluates incoming mail based on authentication, sender reputation, content analysis, and user engagement patterns.

MBPs are responsible for protecting their users from spam, phishing, and malware while ensuring legitimate messages reach the inbox. Their filtering decisions directly impact your deliverability, which is why understanding each provider's unique approach matters for email marketers.