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Diagnosing failed (or deferred) inbound email deliveries

In a hurry? Here's a quick workaround if you don't have time to troubleshoot: Deliver a message to an alternate email address

This article explains how to identify both the status and cause of a message that has failed to deliver to your mail server.

Current delivery status

When a message is shown with a muted yellow background in the Quarantine+ grid, it means that the message has not been delivered to your mail server.

This is an example of what a deferring or failed delivery looks like:

The accompanying icon (in the left column) indicates the reason that the message has not been delivered.

Icon Status
Queued Currently queued for delivery and should arrive in a moment.
DeferringDelivery attempt failed but we'll keep trying. Delivery logs should be available to diagnose the problem.
FailedFailed to deliver to one or more recipients. The delivery logs should be available to diagnose the problem.
Deep filteredMessage is being temporarily held for additional filtering. Use the "Deliver" link to immediately release from the quarantine.

The delivery details

For deferred or failed messages, the Delivery logs will help identify the reason.

Hover over the message and click on the Logs link:

The most important part of the delivery log will be highlighted in bright yellow to make it easy to find:

The three key pieces of information include:

  1. Delivery status
    • sent - The message was successfully delivered to your mail server
    • deferred - The message is trying to deliver but experiencing a problem
    • bounced - The delivery attempt failed and will not be automatically tried again
    • expired - Despite repeated delivery attempts (for several days), the delivery was unsuccessful.
  2. Your mail server and IP address - This identifies the hostname and IP address of the mail server that we attempted to deliver to.
  3. Response from your server - The literal error message that your mail server returned to describe why the delivery failed. The only exception to this is when your mail server is completely unreachable, in which case the message will usually indicate that the "connection timed out". This usually means that your mail server is offline or behind a firewall that is blocking our connection attempts.


  • Every delivery attempt and "status" is highlighted in bright yellow with the latest attempt being at the bottom
  • Messages with multiple recipients will have a delivery attempt and status line for each recipient, as well as an indicator of who that delivery was for. In the example above, the recipient is indicated like this: to=<>

Understanding the problem

The response from your mail server can be difficult to understand. Following are some common error messages and an explanation of what they mean. If you don't see your error listed here, share the delivery log with your mail provider for assistance.

Message Explanation
No Such User Here The recipient email address does not exist on your mail server. Check the recipient address for typos.
Mailbox is full The recipient email box (on your mail server) has too much mail and they need to remove some mail before your server will accept more mail on their behalf.
Message size xxxx exceeds size limit xxxx of server Your mail server rejected the message because it was too large.
Insufficient system resources Usually means your server is low on disk space or ram. See also:
Why is my mail deferring with an "Insufficient system resources" error?
Please turn on SMTP Authentication in your mail client. Your mail server is not configured to accept mail for your domain. This most likely indicates that you need to follow these steps: Configuring cPanel with SpamHero
Hop count exceeded - possible mail loop Messages are being redirected back to our servers after delivery (causing an infinite loop). If you are using Microsoft 365, see: The "Hop Count Exceeded", mail loop error analysis + solutions (Microsoft 365 / Office 365)

Flushing the mail queue

After resolving the issue that is causing mail to defer, our mail server will eventually deliver all messages queued for delivery. Or, you can re-attempt a delivery of all messages queued for delivery within 15 minutes by going to the Tools > Flush mail queue option

Flushing the mail queue will only retry messages that are currently queued for delivery. For bounced and rejected messages, see: View or re-queue messages that failed to deliver

Last updated August 31, 2023