How to distinguish between spam-foldering and rate limiting?

You send a campaign and the numbers look wrong. Opens are down, but you're not sure why. Before you start fixing things, you need to know which problem you're actually dealing with. Rate limiting and spam-foldering look similar from the outside (both kill your results) but they're completely different under the hood.

Rate limiting is when a receiving mail server decides you're sending too fast and pushes back. The server doesn't reject your emails permanently. It defers them, essentially saying "try again later." Your emails still get delivered, just slowly.

Spam-foldering is when your emails arrive at the destination just fine, technically speaking, but the mailbox provider routes them straight to the spam folder instead of the inbox. Delivery speed is totally normal. The mail just ends up in the wrong place.

What to look for in your logs

Your ESP's delivery logs are where you'll find the answer. Here's what each problem looks like.

Rate limiting signs:

• Deferred messages piling up in your log (look for "deferred" or "retry" status)
• 4xx response codes (like 421 or 451) from the receiving server
• Messages that eventually show "delivered" but took hours instead of minutes
• Delivery times stretching across a much longer window than usual

Spam-foldering signs:

• Clean delivery logs with no deferrals or 4xx errors
• Messages showing "delivered" status quickly and normally
• Open rates that have dropped significantly despite normal delivery
• Seed test results showing inbox placement in spam (not delivery failure)

Now the clearest tell is this: if your logs show fast, clean delivery but your open rates have fallen off a cliff, that's spam-foldering. If your logs are full of deferred messages and retry attempts, that's rate limiting.

When you see both at once

Sometimes you'll see deferred messages AND low opens at the same time. That's not unusual. A provider like Gmail might rate limit your sending during a campaign and then, once it does accept the mail, route it to spam. Both problems can exist together, and often the rate limiting is actually a symptom of a reputation issue underneath.

In that case, fix the reputation problem first. Reducing send volume helps with rate limiting short-term, but if spam-foldering is the root cause, volume reduction alone won't get you back to the inbox.

How to fix each one

Rate limiting responds to volume adjustments. Slow your sending cadence, warm up new IPs more gradually, and check whether you're hitting a specific provider's limits.

Spam-foldering needs reputation repair. That means auditing your authentication, cleaning your list, improving engagement, and reducing sends to unresponsive subscribers. The fix takes longer but it's the one that actually matters.

Not sure which problem you're dealing with? Our SOS hotline is free and we're happy to walk through your logs with you.