What Is an Inbox Provider?
TL;DR
An inbox provider, also called a mailbox provider, is the company that runs the receiving mail service for a recipient, such as Google, Microsoft or Yahoo. The provider owns the filtering decision, so its published requirements and its own view of your sending reputation, not your sending tool, determine where a message lands.
Why does the provider matter more than the sending tool?
Because the decision is made on the receiving side. A sending platform can authenticate correctly, pace volume and format cleanly, and every one of those is an input to a model owned and tuned by the receiver. No sending tool can place a message in an inbox; it can only avoid the reasons a provider would not.
This is also why provider documentation is the most reliable source available. Requirements published by Google and Microsoft are the only part of the filtering system that is stated rather than inferred, and everything else circulating as a rule of thumb is inference.
Do the major providers filter differently?
Enough that placement has to be read separately for each. Google publishes sender requirements and exposes reputation data through Postmaster Tools. Microsoft publishes its own programmes and exposes IP-level data through Smart Network Data Services, and its corporate tenants often add a gateway with policies stricter than the defaults.
For business outreach those two dominate what you will encounter, since most work addresses resolve to Google Workspace or Microsoft 365. A message that lands at one and not the other is normal and points at the difference between their models rather than at a fault in the send.
Frequently asked questions
By the MX records on the recipient domain, which name the receiving servers. Google Workspace and Microsoft 365 domains are recognisable from those records, and knowing the split across a list tells you which requirements and which seed mailboxes are worth the effort.