How can I manage throttling and greylisting?
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Throttling is when a mailbox provider says "slow down" by temporarily deferring your messages. Greylisting is a specific flavor of that. The receiving server says "I don't recognize you, try again in a few minutes" and rejects the first attempt with a 4xx response. Legitimate mail servers retry. Spam bots usually don't.
Both are normal. What you're actually managing isn't the throttle itself. It's the reputation and infrastructure that tell providers whether to keep throttling you or let you through.
1. Make sure your MTA retries correctly
Greylisting only works as a filter if senders retry politely. Any reputable ESP handles this automatically. If you're running your own Postfix or similar, confirm retry intervals are sensible: 5 minutes, then 15, then longer. Don't hammer a server that just told you to wait.
2. Authenticate properly
Providers throttle less aggressively when they can verify who you are. Get SPF, DKIM, and DMARC set up and passing. You can check all three at once by pasting a delivered header into our email header analyzer.
3. Warm up new domains and IPs
A fresh sending domain with zero history gets throttled hard. Ramp up volume over 2 to 4 weeks. Day 1: a few hundred sends to your most engaged subscribers. Day 7: a few thousand. Day 14: broader audience. Big volume spikes look like a compromised account, and providers respond by slamming the brakes.
4. Keep your list clean
Throttling often follows a rise in bounces or complaints. Sending to dead addresses and disengaged readers raises both. Remove hard bounces immediately. Segment out people who haven't opened in 6 months. If your list hasn't been validated recently, our list cleaning catches the stale addresses before they drag your reputation down.
5. Read the deferral codes
But when a message is deferred, the 4xx response usually tells you why. "Too many connections" means slow your concurrency. "Policy reasons" points at content or reputation. Your ESP's logs should surface these. If they don't and you're stuck, the SOS hotline is free.
The shortest version: send consistently to people who want your mail, authenticate everything, and let your MTA retry cleanly. Greylisting disappears on attempt two. Throttling eases as reputation builds.
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