An MTA accepts outbound mail, queues it, resolves the destination via MX records, and delivers it over SMTP to the receiving server, handling retries, bounces, and rate limiting along the way. It's distinct from a mail client (MUA) or a mailbox store (MDA).
Operating an owned MTA gives direct control over queue behavior, retry policy, and per-domain routing, at the cost of also owning IP reputation management, bounce handling, and blocklist monitoring that a third-party API would otherwise absorb.
PulsePigeon runs its own MTA infrastructure specifically to control lane isolation and routing decisions at the queue level, rather than relying entirely on a third-party sending API.