Mailbox provider
A company that hosts email mailboxes for people or organisations, such as Gmail, Outlook.com, Yahoo Mail or iCloud Mail, and decides what reaches them.
What it means
RFC 6449 defines a mailbox provider as a company or organisation that provides email mailbox hosting for end users and organisations, many of which are also access providers. M3AAWG uses the same term, abbreviated MBP, and RFC 6449 treats the word ISP as meaning either an access provider or a mailbox provider.
Mailbox providers include consumer webmail services (Gmail, Outlook.com, Yahoo Mail, iCloud Mail), business hosting (Google Workspace, Microsoft 365) and many regional and corporate systems, some behind secure email gateways. Each runs its own filtering, sets its own requirements and keeps its own sender reputation data.
To know which provider hosts a recipient domain, look at its MX records. Business domains on Google Workspace or Microsoft 365 are filtered by those providers' business systems, not their consumer rules.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
All 432 terms in the glossaryThe wiki: blocklists and bounce codes