What’s a “quarantine” or “limbo” folder classification?

You sent an email to a corporate contact. Your ESP says it delivered. No bounce. No error. But your contact never got it. What happened? It's probably sitting in quarantine.

Quarantine (sometimes called "limbo") is a holding state used by enterprise email security gateways. When a filter isn't confident enough to accept or reject a message outright, it parks the message in a queue for an administrator to review. The recipient never sees it unless an admin manually releases it.

This is almost exclusively a B2B problem. Consumer inboxes like Gmail or Yahoo Mail don't quarantine this way. But corporate environments running tools like Barracuda, Proofpoint, or Mimecast do it constantly. Some organizations have quarantine policies so strict that even legitimate, authenticated emails get held.

Why quarantine is tricky to detect

The reason it catches so many senders off guard is that quarantine doesn't generate a bounce. From your ESP's perspective, the message was accepted by the receiving mail server. Technically, delivery succeeded. But it never reached the inbox. You get a green checkmark on a message your recipient will never read.

This makes it much harder to diagnose than a standard hard bounce. At least a bounce tells you something went wrong.

How to detect it

• Ask your contact directly. If you're not getting replies from a specific company domain, email from a personal account and ask if they're receiving your messages. Simple, but it works.
• Check your engagement data by domain. If you have a cluster of contacts at one company with zero opens across multiple campaigns, quarantine is a strong candidate.
• Use seed testing with corporate addresses. Sending test emails to a real corporate mailbox you control (ideally with a strict gateway in place) can confirm whether your messages get held.
• Look at your header data. If a message does eventually get released, the email headers will often show which gateway held it and for how long. Our free email header analyzer can pull those details out for you.

What actually causes it

Corporate gateways score incoming messages against a combination of signals. Authentication failures (missing or broken SPF, DKIM, or DMARC) are a fast path to quarantine. So are IP reputation issues, bulk sending patterns, and content that pattern-matches to phishing templates. Some organizations also quarantine anything from a domain that hasn't previously emailed their users, regardless of how clean the sender looks.

What to do about it

• Make sure your authentication is solid. SPF, DKIM, and DMARC all aligned and passing is your baseline. No gateway exceptions here.
• Ask your contact's IT team to allowlist your sending domain. It feels awkward to ask, but for important B2B relationships it's completely normal. Give them your sending domain and the IP ranges your ESP uses.
• Avoid bulk-sending patterns to corporate domains. Sending to 50 contacts at the same company in one batch looks a lot like a phishing sweep. Stagger those sends.
• Warm up new domains before blasting enterprise addresses. Brand new sending domains with no reputation history get held more aggressively.

If you're regularly losing emails to quarantine across multiple B2B accounts and you're not sure where the problem starts, it's worth a quick check on your domain's reputation. Our blocklist checker is free, and if things look broken, our SOS hotline is there for exactly this kind of situation.