Mailbox Provider
The company that operates a recipient's email, such as Google or Microsoft, and decides whether your message lands in the inbox or spam.
A mailbox provider is the company that runs a recipient's email service and decides what happens to each incoming message. In B2B, the two that matter most are Google Workspace and Microsoft 365, with a long tail of others. The mailbox provider is the gatekeeper: it evaluates every message and routes it to the inbox, the spam folder, or a silent block. Mailbox providers are sometimes called inbox providers or, more technically, MBPs.
Why it matters for outbound
No outbound message reaches a buyer without the mailbox provider's consent. These systems weigh authentication, sender reputation, engagement, and complaint signals to decide placement, and Google and Microsoft each apply their own logic and thresholds. A campaign that performs well at one can struggle at another, which is why inbox placement has to be earned provider by provider, not assumed.
What providers evaluate
- Authentication: valid SPF, DKIM, and DMARC
- The sending history and reputation tied to the domain
- How recipients engage: opens, replies, and crucially complaints
- Bounce rate and signs of poor list quality
How we earn placement
As a managed agency, we treat each mailbox provider as a relationship to be maintained, not a wall to be beaten. Our deliverability and inbox placement work tunes authentication, reputation, and list quality to each provider's expectations, and monitors placement so a drift at Microsoft or Google is caught and corrected before it costs pipeline. The goal is simple: consistent inbox access where your buyers actually read mail.
From definitions to pipeline
Outword turns outbound theory into a running motion. Book a call to see what that looks like for your team.