Three families of MTAs in 2026:
1. General-purpose (Postfix, Exim, Sendmail). Receive, queue, deliver. Fine for a few thousand emails/day. Hard to tune for huge volume — no native per-destination concurrency, weak observability.
2. Sending engines (KumoMTA, PowerMTA, Halon, MailerQ). Built for outbound only. Per-destination queues, IP pools, warmup curves, bounce processing, FBL ingestion. This is the bulk-sender category.
3. Cloud APIs (SendGrid, Postmark, Amazon SES). You hand off via HTTPS or SMTP-relay. They handle MTA + IPs + reputation. Easy at low scale, expensive at high scale, less control.
Where Kumo wins:
- You're shipping >100k msgs/day and SendGrid pricing hurts.
- You need per-domain throttling that respects Gmail's 250/sec or Yahoo's 100/sec.
- You want to own your IPs and reputation.
- You want hot-reload config without restarts.
Where you should NOT use Kumo:
- You ship <10k msgs/day. Use SES or a Postfix relay; the operational overhead isn't worth it.
- You also need to receive mail. Pair with Postfix or just don't.
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