Raising the concurrent connection limit on your MTA does not scale email. Mailbox providers apply per-IP and per-domain rate limits. If you outrun them you collect deferrals, then blocks.
Separate the queues
Transactional mail should not wait behind a newsletter. Use different queues, and often different IPs or subdomains, so a slow campaign cannot delay password resets. Retry 4xx with backoff. Do not hammer a 421 with the same burst.
Add IPs the slow way
New IPs need warmup. Spreading volume across ten cold addresses is worse than one warm IP. Watch per-provider acceptance. Microsoft and Gmail will not ramp on the same curve.
Application-level brakes
Cap new-user mail, throttle on bounce spikes, and kill a campaign if complaint webhooks fire. Scaling SMTP is an operations problem: dashboards, on-call, and the willingness to stop sending.
