What are “451 4.7.1” throttling messages?

Your campaign just sent, but a chunk of messages aren't getting through. The bounce log keeps showing 451 4.7.1. Is this something you broke, or just a blip? Good news: it's almost certainly temporary. Here's what it means.

A 451 is a 4xx soft bounce code. The receiving server accepted the connection but pushed your message back with a "not right now" signal. The 4.7.1 part narrows it down to a policy or security concern. Together, they mean the receiving server is suspicious of your message or your sending pattern and wants to hold off before accepting it.

You'll typically see messages like these in your bounce logs:

• "451 4.7.1 Service unavailable, try again later."
• "451 4.7.1 Temporarily rejected due to policy."
• "451 4.7.1 Please try again."

These show up in a few common scenarios. You might be hitting greylisting, where the receiving server deliberately bounces first-time senders to check if the mail server retries properly (which spambots usually don't). You might also be getting throttled because your sending volume jumped suddenly, your complaint rate ticked up, or the receiving server simply doesn't recognise your IP yet.

The important distinction is what this code is not. A 550 5.7.1 is a permanent rejection. That means the server has decided it doesn't want your email and you should stop retrying. A 451 4.7.1 is the opposite. Retrying is exactly what you're supposed to do. Your ESP handles this automatically with standard backoff logic, spacing retries over minutes or hours.

Most 451 4.7.1 deferrals clear up on their own within a few hours. If they keep happening over multiple days, that's the signal worth paying attention to. Persistent deferrals from the same provider usually point to a reputation issue worth investigating properly: check your complaint rate, review any recent volume spikes, and confirm your authentication records are solid.

Still if you're seeing a 20% drop in open rates from a specific provider alongside these codes, that's a sign the throttling is ongoing rather than a one-off. Worth digging into your sending history with that provider to see if something changed recently.

You can run a quick check on your domain's reputation with our free Blocklist Checker. If things feel more urgent, our SOS hotline is free and we actually pick up.