DELIVTERMS: The (almost) weekly series here on Spam Resource that defines deliverability terminology. Today, I'm going to talk about Internet Service Providers (ISPs) and Mailbox Providers (MBPs).

ISPs and MBPs: Talk to any deliverability consultant and you've likely heard the two terms thrown around. What do these acronyms actually mean?

ISP means Internet Service Provider, and you'll find me guilty of using this as a catch all term for any place that a big email sender would be sending email messages to. Even though I include Gmail and Yahoo, on this list, for example, that's a bit inexact in that true ISPs are companies that provide internet access to their subscribers. Think Comcast and Spectrum.

MBP means Mailbox Provider, and that is perhaps a bit more accurately used as a catchall term for any service that provides email addresses for users to receive mail at. How you measure who the largest provider is can vary, but most of the ways I've done it tend to suggest that Google's Gmail is the biggest mailbox provider out there, with most of the senders I deal with having the majority of their subscribers be Gmail users.

Bonus terms: B2C and B2B mailbox providers.

B2C means "business to consumer" and when saying B2C subscribers, we're using a shorthand that means anyone who typically subscribes from a personal (non-business) email address or provides a personal email address, maybe when signing up for Netflix, ordering food from Uber Eats, or buying widgets for the home from an online store. (I'm also occasionally guilty of using the acronym DTC to refer to this type of activity -- meaning "direct to consumer.") In the US B2C subscriber space, the top five providers (Gmail, Yahoo, Hotmail, Comcast, Apple) make up over 90% of most B2C marketer email lists. There are lots of other mailbox providers out there, but their reach is below 1%, often far below 1%, at that point.

Over here, you'll find where I've published domain lists for top B2C ISPs, including Microsoft Hotmail/, AOL/Yahoo, Gmail, Comcast, AT&T, and others.

B2B means "business to business" and when saying B2B subscribers, we're using a shorthand that means anyone who typically subscribes from a business email address, whether that be for purchasing widgets for the office or any other kind of business-related communication. There's a bit of ambiguity there in that some people do use Gmail, Hotmail and Yahoo accounts for work purposes, but generally what we're referring to is custom email domains, not the usual ones. Over here, I've listed some of the top players in the B2B email hosting space, and I explain how to tell what provider hosts mail for a given domain.

Want to learn more about deliverability terminology? If so, be sure to visit the DELIVTERMS section here on Spam Resource.



  1. Interesting, I use MSP for Mailbox Service Provider (like ESP Email Service Provider)


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