Also called: ESP
An email service provider (ESP) is the platform that actually hosts and sends the mail, Google Workspace, Microsoft 365, or a dedicated sending infrastructure provider, as distinct from the sequencer that schedules and tracks the sequence running on top of it.
An ESP is the layer that physically sends the mail: the mailbox and domain infrastructure, not the tool scheduling what goes out and when. For cold outbound, that's usually a real Google Workspace or Microsoft 365 tenant, or a dedicated provider that provisions pre-warmed inboxes on fresh domains built specifically for sending.
Mail sent from an ESP-native tenant, a real Google Workspace or Microsoft 365 account, sits inside that provider's own trust graph rather than routing through third-party SMTP. That distinction is part of why teams running cold outbound at real scale care which kind of ESP setup sits underneath their sequencer, not just which sequencer they picked.
A sequencer and an ESP are two separate purchases solving two different jobs, even though they sit next to each other in a stack. The sequencer decides what to send and when. The ESP is where the mailbox actually lives and where its sending reputation gets built, one send at a time.
Price the ESP and the sequencer separately when building a stack, since neither one substitutes for the other, and a strong sequencer connected to poorly chosen mailbox infrastructure still inherits whatever reputation problems that infrastructure has.
People use 'ESP' loosely enough to mean the sequencer, the mailbox provider, or the whole sending stack depending on who's talking. The distinction matters here specifically because a cold outbound program usually needs both, bought from different places, doing different jobs.
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