SMTP

TL;DR A mail transfer agent (MTA) is the server software that moves email from sender to recipient across SMTP hops — the layer where queue policy, retry logic, throttling, and authentication decide whether your message reaches the inbox or quietly collapses to spam. Most foreign-trade teams below 50,000 messages per month should start with a managed cloud relay (SES, Postmark, Mailgun, Resend) and move to self-hosted Postfix or Exim only when they hit a compliance wall, a deliverability ceiling, or a volume tier where the per-message cost crosses a clear threshold. Treat deliverability as a CI problem: per-domain pacing, 72-hour retry cap, 5 soft bounces in 7 days suppression, and continuous SPF/DKIM/DMARC alignment checks. ...