Mailbox provider

A mailbox provider is the company or system that hosts the recipient's inbox: consumer services such as Gmail, Outlook.com and Yahoo Mail, business platforms such as Google Workspace and Microsoft 365, and the many smaller hosts and self-run servers. You can usually tell which one a business uses from its MX records.

Each provider runs its own filtering. It decides whether to accept a message at all (the SMTP stage), then whether to put it in the inbox, a tab or the spam folder, using authentication, sender reputation, content and how recipients engage. Many businesses also put a security gateway in front of their provider, which adds its own filtering.

Rules differ by provider. Google and Yahoo publish sender requirements covering authentication, one-click unsubscribe and spam rates; Microsoft publishes its own guidance and postmaster tools. B2B lists are dominated by business email platforms, so check how your outreach performs per provider rather than as one average.

Providers also differ in how they answer mailbox checks. Some reject unknown recipients immediately; others accept everything at first and bounce later, which makes the domain behave like a catch-all.

Skrap.ai's verifier talks to each domain's mail server directly, so results reflect how that provider actually answers. When a provider accepts everything, you get Risky rather than a false Valid; when it won't give an answer, you get Unknown, which is free. Try it in the free email verifier.