Mailbox provider
The company providing the recipient's email service — Gmail, Outlook, Yahoo, Apple, etc. Each has its own filtering and reputation system.
A mailbox provider (MBP) is the company providing the recipient's email service — Gmail (Google), Outlook (Microsoft), Yahoo, Apple iCloud, and the long tail of corporate and ISP-operated mail systems. Each MBP runs independent filtering and reputation systems; your sender reputation at Gmail has nothing to do with your reputation at Outlook.
For deliverability work, the four MBPs that matter most for B2C are Gmail (dominant in personal email globally), Apple Mail (large iOS share, MPP impact), Outlook/Hotmail (dominant in corporate and older consumer email), and Yahoo (large legacy consumer base, AT&T email runs on Yahoo infrastructure). For B2B, add Microsoft 365 corporate (independent of Outlook.com consumer), Google Workspace corporate, and a long tail of corporate email systems.
Monitoring per-MBP reputation is essential. Google Postmaster Tools exposes Gmail-side reputation metrics; Microsoft SNDS exposes Outlook-side data. Yahoo and Apple don't publish equivalent dashboards, so monitoring there relies on seed-list testing or panel-based services.