PowerMTA Rate Limiting & Throttling
Send too fast and mailbox providers throttle or block you. PowerMTA's per-ISP rate limiting is how you send as fast as possible without crossing the line.
TL;DR
Set per-domain limits (max-msg-rate, max-smtp-out) so each provider gets mail at a safe pace. Too aggressive triggers 4xx throttling; too slow wastes capacity. Tune per ISP and watch deferrals.
Why throttling matters
Gmail, Outlook and Yahoo each enforce their own acceptance rates. Exceed them and you get 4xx deferrals or temporary blocks. Throttling is how you respect those limits while staying fast.
Key directives
<domain gmail.com>
max-msg-rate 120/min
max-smtp-out 20
</domain>- max-msg-rate — messages per minute to that domain.
- max-smtp-out — simultaneous connections.
These live in your PowerMTA config and can be set per ISP.
Finding the right numbers
There's no universal value — it depends on your IP reputation and warmup stage. New IPs need conservative limits (see the warmup schedule); established IPs can push harder. Start low, watch deferrals, increase gradually.
Reading the signals
Rising 4xx deferrals mean you're hitting limits — back off. A clean queue with high throughput means you have headroom. Monitor via PowerMTA logs.
Per-ISP rules of thumb
Different providers tolerate different speeds. Maintaining separate rules per major mailbox provider — and adjusting as reputation grows — is exactly the kind of tuning a panel simplifies.
AutoPMTA manages per-ISP throttling visually and adapts limits as your reputation grows. View plans.
Key takeaways
- Throttling respects each provider's acceptance rate.
- Tune max-msg-rate and max-smtp-out per ISP.
- New IPs need conservative limits; ramp with warmup.
- Watch 4xx deferrals as your tuning feedback signal.
Frequently asked questions
What is email throttling?+
Throttling is deliberately limiting how fast you send to each mailbox provider so you stay within their acceptance rates and avoid 4xx deferrals, temporary blocks or reputation damage.
What is a good max-msg-rate in PowerMTA?+
There's no universal value — it depends on your IP reputation and warmup stage. Start conservatively on new IPs, monitor 4xx deferrals, and increase the rate gradually as reputation builds.
Why am I getting 4xx errors when sending?+
4xx responses are temporary deferrals, usually meaning you're sending faster than the provider accepts. Lower your rate limits per ISP and the deferrals should ease.