What spooling is
Spooling (sometimes called mail bagging or queueing) means holding mail temporarily when it can't be delivered yet, then delivering it later.
Because your MX records point to our filtering servers, senders hand their mail to us, not directly to your mail server. If your server is down, the mail has already been accepted. We keep it in a queue and keep trying to deliver it. When your server is back, the held mail is delivered.
How it works, step by step
- A sender delivers a message to our filtering servers, as normal.
- The message is filtered. Clean mail is ready for delivery.
- We try to deliver it to your mail server. Your server doesn't respond, or responds with a temporary error.
- The message is held in a queue for up to 5 days, with delivery retried.
- When your server accepts mail again, the queue is delivered.
Without and with spooling
Email was designed to cope with outages. RFC 5321, the SMTP standard, says sending servers should retry when delivery fails temporarily, and suggests giving up after at least 4–5 days. So why does spooling help?
| Item | Without spooling | With iBlockSpam spooling |
|---|---|---|
| Who holds the mail | Each sender's own server | Our filtering servers |
| Retry schedule | Set by each sender; varies widely | Consistent, up to 5 days |
| Senders who give up early | Mail may bounce back to them | Already accepted, so it waits for you |
| Bounce messages to your contacts | Possible during a long outage | Avoided while within the 5-day window |
| Visibility | None until mail arrives | Message history in the filtering panel, opened from your dashboard |
In practice, many senders don't follow the suggested retry period. Some bounce after hours. With spooling, mail is accepted once and held in one place.
When spooling helps
- Mail server outages: hardware failure, a crashed service or a full disk on a self-hosted server.
- Planned maintenance: server upgrades, reboots or moving a server to a new host.
- Network problems: a lost connection at your office or data center.
- Platform incidents: your hosted mail provider has a problem accepting inbound mail.
Honest limits
- 5 days is the maximum. If your server is down longer than that, held mail can't be kept indefinitely. For long migrations, plan the cutover rather than relying on the queue.
- Spooling isn't backup. It holds mail in transit until it's delivered. It isn't an archive of delivered mail. Message history is kept for 30 days for tracing, not for restoring mailboxes.
- Spooling can't help if mail doesn't reach us. If your MX records are changed to point elsewhere, mail no longer flows through iBlockSpam. Domain monitoring alerts you to that kind of change.
- A permanent rejection isn't an outage. If your server rejects mail permanently (for example, "user unknown" or a misconfigured connector refusing our servers), the message isn't held as if your server were down. That's why setup includes a verify step.
During and after an outage
- During: open the filtering panel from your dashboard and check the message history to see that mail is arriving and being held.
- After: held mail is delivered automatically. Large queues may take a little while to clear.
- If you're moving servers: update the delivery destination in iBlockSpam before you switch off the old server, so held mail goes to the right place.
Related
Frequently asked questions
How long will you hold my mail?
Up to 5 days. If your mail server is unreachable, inbound mail that passed filtering is held in a queue and delivered when the server accepts mail again. If an outage lasts longer than 5 days, held mail can no longer be kept, so plan major migrations with that window in mind.
Do I need to do anything to get spooled mail?
No. Once your mail server is reachable and accepting mail again, held messages are delivered automatically. They may arrive over a short period rather than all at once, and their original sent dates are preserved in the messages.
Doesn't email retry on its own anyway?
Yes. Under RFC 5321, sending servers retry temporarily failed deliveries, typically for a few days. But each sender has its own retry schedule, and some give up early or bounce. With spooling, mail is accepted and held in one place, so you aren't relying on every sender's settings.
Related guides
What happens to email when your mail server is down?
Senders retry when your mail server is down, but not forever. How SMTP retries work, 4xx vs 5xx errors, what senders see, and how outage spooling helps.
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What an MX record is, how priority values pick the server that gets your mail, why backup MX can bypass a spam filter, and how to check your own records.
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A cutover checklist for MX changes: lower TTL, stage the new destination, keep the old server accepting mail, verify with headers and roll back cleanly.
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