How can ESPs detect and pause bad senders?

If you've ever had a campaign suddenly stop mid-send without warning, there's a good chance your ESP's abuse detection kicked in. ESPs protect their shared infrastructure constantly, and when something looks wrong, they don't wait to find out if it's a mistake.

Here's what they're watching for.

Hard bounce spikes. A sudden jump in hard bounces tells the ESP that your list is low quality or outdated. Most ESPs start flagging accounts when hard bounce rates climb past 5%. Some will pause sending automatically before a human ever reviews the account.

Complaint rates. Spamhaus and mailbox providers route spam reports back to ESPs through feedback loops. If your complaint rate crosses 0.1% to 0.3%, that's a red flag. Above 0.3% and most ESPs will act fast, because complaints hurt every sender sharing that IP pool, not just you.

Spam trap hits. Spam traps are addresses used specifically to catch bad senders. ESPs watch for them directly or get reports from blocklist operators. Hitting one isn't always intentional, but it does signal a list hygiene problem worth taking seriously.

Volume anomalies. Sending 500 emails a day and then suddenly sending 50,000 looks suspicious. Abuse detection systems flag dramatic volume spikes, unusual timing patterns, or sharp drops in engagement metrics.

Blocklist appearances. If a sending IP gets listed on a major blocklist, the ESP traces back which account triggered it. That account gets flagged immediately, even if the listing happened on their end and not yours.

When detection fires, ESPs have a range of responses they can apply.

• Throttling slows your send rate to limit further damage while someone investigates.
• Pausing stops the account entirely until the issue is reviewed.
• IP pool isolation moves your sending to lower-reputation IPs so other senders on the shared pool don't suffer.
• Account termination happens when violations are repeated or severe. That one's harder to come back from.

But most ESPs don't jump straight to termination for a first offense. If you're a legitimate sender who hit a rough patch (stale list, a campaign that went out to the wrong segment, an import gone wrong), the usual path is a pause, a conversation, and a remediation plan. What gets accounts terminated is the pattern, not a single bad day.

The best protection is to not give detection systems anything to find. Clean your list before large sends, watch your complaint rate actively, and warm up new sending infrastructure gradually. If your list feels stale, getting it cleaned before a campaign is cheaper than recovering from a paused account. (We do that, if you need it ;))

Worried your current list might trigger flags? Review My Emails validates your list before it causes a problem, not after.