How do MBPs differ from ESPs?
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Your ESP sends your emails. Your MBP decides what happens to them when they arrive. That one sentence is the whole difference, but understanding it changes how you troubleshoot deliverability problems.
An Email Service Provider (ESP) like Mailchimp, Klaviyo, or Brevo is the platform you use to build, send, and track your emails. It manages your list, your templates, your sending infrastructure, and your analytics. When you hit send, your ESP's job is done.
A Mailbox Provider (MBP) like Gmail or Outlook owns the inbox on the other end. It receives your email, runs it through spam filters and reputation checks, and decides where it goes: inbox, spam folder, or rejected entirely. The MBP answers to its users, not to you.
That split matters enormously when something goes wrong. If your emails aren't arriving, your ESP can confirm whether the message was sent and accepted. But if the MBP is filtering it to spam or blocking it, your ESP can't override that. The MBP's decision is final, and it's based on your sender reputation, your authentication setup (SPF, DKIM, DMARC), and how your past recipients have engaged with your mail.
Think of it this way. Your ESP is the courier that picks up your package and drives it to the door. The MBP is the building's mail room that decides whether the package gets upstairs or gets held. Two completely separate operations.
Practically speaking, this means you need both sides working well. A great ESP with a damaged sender reputation still lands in spam. And a strong reputation won't help if your authentication is broken and the MBP can't verify you're who you say you are.
If you're trying to figure out why Gmail and Outlook behave differently for the same campaign, that's a good next read.
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