MTA (Mail Transfer Agent)

An MTA is the engine layer of email transmission — it handles the SMTP conversation, queues messages, retries on failure, and routes outbound mail. ESPs run hardened MTAs at scale; you generally don't want to run your own for cold outreach.

When people say "shared MTA reputation" they mean: every customer using the same ESP's MTA cluster shares the IP-level reputation of that cluster. One bad actor can degrade the deliverability of everyone else on the same IPs.