What’s “policy quarantine” in some regional ISPs?

You hit send, your ESP shows no bounce, but your recipient swears they never got the email. No error, no rejection, just silence. That's what policy quarantine feels like from the sender's side, and it's one of the more frustrating things to troubleshoot.

Policy quarantine is when a regional ISP holds your message in a suspended state instead of delivering it or bouncing it back. It's not a hard reject. It's not a spam folder. It's a deliberate pause where the ISP is essentially saying "we're not sure about this one yet." The message sits in a queue on their side, sometimes for hours, sometimes for days, until a postmaster reviews it or an automated process makes a decision.

The tricky part is that you get nothing from your sending logs that tells you this is happening. No 5xx error. No soft bounce. Just a successful SMTP handoff that never turns into a delivery.

How to detect it

Start by comparing what your sending data tells you against what your recipients are actually seeing. If you sent to 1,000 addresses at a regional ISP and zero bounced but your open rate is suspiciously low, something is holding your mail. Seed testing helps here. Send to a test address at that ISP domain before your main send and check whether it arrives, and how long it takes.

Delivery timestamps are your other clue. If messages are arriving 4, 8, or 24 hours after sending (and your delivery data shows provider-specific delays), that's a quarantine pattern, not just general slowness. Normal delivery to a healthy inbox happens in seconds to minutes.

You can also check your email headers. When a quarantined message finally does arrive, the Received headers will often show an unusual gap between the sending timestamp and the final delivery timestamp. That gap is the quarantine window.

Why regional ISPs do this

Regional ISPs don't always have the same automated filtering infrastructure as Gmail or Outlook. Instead of making instant pass/fail decisions, some of them build in a holding period when a sender looks unfamiliar, sends a volume spike, or has authentication they haven't seen before. It's a manual fallback that compensates for less sophisticated filtering.

But this is more common in parts of Eastern Europe, Southeast Asia, and Latin America, where some ISPs still run smaller-scale mail infrastructure. The rules are less standardized than major providers and can vary quite a bit from one ISP to the next.

How to avoid getting stuck there

A few things reduce your chances of hitting policy quarantine in the first place:

• Warm up slowly to unfamiliar ISPs. Volume spikes are one of the biggest quarantine triggers. If you've never sent to a particular regional ISP before, start small.
• Check your authentication is clean. SPF, DKIM, and DMARC should all pass. ISPs reviewing suspicious mail will look at these first. You can run a quick check with our free SPF checker.
• Keep your list clean. High invalid-address rates make you look like a bad sender to any ISP. The more bounces and unknown users you're hitting, the more suspicious your traffic looks.
• Monitor your sending reputation. Check whether your domain or IP is listed on blocklists with our blocklist checker. A listing at a regional blocklist can trigger quarantine even when major blocklists show you as clean.

How to contact the ISP directly

Here's the thing: most regional ISPs do have postmaster contacts, but they're not always easy to find. Start with a WHOIS lookup on the domain to find abuse or postmaster contact details. Many publish an abuse@ or postmaster@ address on their domain. If you can find their postmaster portal (some regional providers have them), that's even better.

So when you reach out, keep it simple and factual. Give them your sending IP, your sending domain, a sample message ID, and the dates you sent. Ask specifically whether mail from your domain is being held for review. Avoid anything that reads like a sales pitch. Postmasters respond to data, not to assurances about how great your newsletter is.

If you're not getting anywhere on your own, our SOS hotline is free. We've dealt with enough regional ISP quirks to know which doors to knock on, and we won't pitch you anything while we help.