What is a mailbox provider (MBP)?
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A mailbox provider (MBP) is the company that runs the servers your recipients' email accounts live on. They accept inbound mail over SMTP, decide whether it lands in the inbox, the spam folder, or gets rejected outright, and they store it so the user can read it later in Gmail.com, the Outlook app, Apple Mail, or whatever client points at their account.
The big four by global volume are Gmail, Yahoo (which also runs AOL), Microsoft (Outlook.com, Hotmail, Live, plus Office 365 for business), and Apple iCloud. Together they handle the vast majority of consumer inboxes. Then there is a long tail: Yandex and Mail.ru in Russia, GMX and Web.de in Germany, Free.fr and Orange in France, Naver and Daum in South Korea, QQ and 163 in China, plus thousands of business domains hosted on Google Workspace or Microsoft 365. See the major mailbox providers globally for the full breakdown.
A mailbox provider is not the same thing as an email service provider (ESP). An ESP like Mailchimp, Klaviyo, or HubSpot is the tool you use to send mail. The MBP is the system on the other end that receives it and decides what to do with it. We cover the distinction in detail in how MBPs differ from ESPs.
What an MBP actually does
Four jobs, roughly in order each message goes through:
- Accept or reject the connection. Your sending IP gets checked against blocklists and the provider's own reputation data before the SMTP conversation even completes. Bad reputation, you get a 5xx rejection at the door.
- Authenticate the message. SPF, DKIM, and DMARC get evaluated. Gmail and Yahoo both require all three for any sender pushing more than 5,000 messages a day to their users, per the 2024 bulk sender rules.
- Score it for spam, phishing, and malware. Content analysis, link reputation, attachment scanning, plus machine-learning models trained on what their users mark as spam.
- Decide placement. Inbox, Promotions tab, Spam folder, or quiet drop. Gmail in particular has gotten more aggressive about silent filtering for low-engagement senders.
Why this matters for you as a sender
Every MBP has its own filtering logic, its own reputation system, and its own thresholds for what counts as acceptable. A campaign that hits the inbox at Gmail can land in Spam at Outlook on the same send, because the two providers weigh different signals. Gmail leans heavily on per-user engagement and domain reputation. Microsoft still gives meaningful weight to IP reputation and SmartScreen content scoring. Yahoo sits somewhere between them.
This is why "deliverability" is never one number. It's a per-MBP scorecard, and you need to read it that way. See why different MBPs produce different inbox results for the same campaign and how mailbox providers filter and classify emails for the mechanics.
The practical takeaway: when you ship a campaign, you are not sending to one inbox system. You are sending to four to six major ones plus dozens of smaller regional providers, each with their own rules. Build your sending practices to satisfy the strictest of them (which is currently Gmail), and the rest tend to follow. Industry guidance from M3AAWG's sender best common practices lays out the baseline every MBP expects: authenticated mail, clean lists, working unsubscribe, low complaint rates.
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