MTA vs ESP

An MTA is software you run; an ESP is a service you rent. An ESP operates MTAs, IP addresses, and deliverability teams for many customers and charges per message or per contact; running your own MTA trades that fee for infrastructure control, data ownership, and flat costs, in exchange for operating responsibility. High-volume senders often cross over when per-message fees exceed the cost of a MailOps team, when they need message data to stay in their own infrastructure, or when shared-pool reputation limits their deliverability.