This article provides a workaround for dealing with this common Microsoft 365 bounce error:

This is most commonly cause caused by a known Microsoft 365 / Office 365 design flaw that has been around for years.
 

What is causing this error?

The underlying problem is that Microsoft 365 sometimes accepts an inbound message and then instead of delivering to the local recipient, treats it as an outbound message. As a result, the message is relayed back to SpamHero, where it is processed and re-delivered to Microsoft (causing a loop until the message bounces).

For more details and alternate solutions, see:

• The "Hop Count Exceeded", mail loop error analysis

Frequently Asked Questions

• Why is this solution referred to as a "workaround"?

  Since the problem is happening inside of Microsoft's network, our options are limited. Typically, the problem is caused by the senders' configuration so the best we can do is provide a way to work around the problem.

  You might also ask the sender to fix their configuration by having them review the following article:

  Getting "Hop Count Exceeded" bounce error when sending to a SpamHero user

• How does this workaround solve the problem?

  On Microsoft's network, the onMicrosoft.com domain is always treated as "local". So even when Microsoft confuses an "inbound" delivery as an "outbound" message, the MX records point to a local destination host.

• Are there any known side effects of this workaround?

  No, the onMicrosoft.com email address is completely invisible to the recipient and the rest of the world (email replies still come from the user's primary email address).

• Can I just change my SpamHero Delivery Host to the onMicrosoft fallback domain?

  Unfortunately, not. The onMicrosoft.com domain is not configured to work as a destination mail host. In other words, this workaround must be setup for each individual recipient address that's experiencing this problem.

Recipient-based Solution

Since this is caused by a bug inside of Microsoft's network, the workaround options are somewhat limited. However, through testing, we have identified a per-recipient address solution that works. The workaround is to deliver messages via the affected recipient's onMicrosoft.com fallback domain. In most cases, the alias address already exists.

Microsoft 365 admin center

Most email users should already have a "fallback" email alias address that ends with [yourSubDomain].onmicrosoft.com (the sub-domain is usually based on your company name or your primary domain name). Use the following steps to find the fallback email alias address for your user:

1. Go to the Active users page in the Microsoft 365 admin center: https://admin.microsoft.com/Adminportal/Home#/users
   If preferred, you can navigate there on your own
   1. Login to your Microsoft 365 admin center (https://admin.microsoft.com/)
   2. Expand the side navigation menu (click the three "horizontal lines" in the upper-left corner)
   3. Click Users > Active users in the side navigation menu.
2. Find and click on the recipient user that is having trouble receiving messages. example

   If needed, use the Search box in the upper-right corner:

   
   

3. In the Aliases section of the user details, make a note of the email alias address that ends with .onmicrosoft.com
    

   Example:

   
   

 
If the user does not have an onMicrosoft alias address...

If the user doesn't have an email alias that ends with .onmicrosoft.com then you will need to create one:

1. Click on the Manage username and email link to find the Aliases control.
2. Enter a Username to match the user's Primary username
3. Click on the Domains listbox and select the one that ends with .onmicrosoft.com
4. Click on the Add button
5. Click on the Save changes button, in the bottom-left corner.

SpamHero control panel

In the SpamHero control panel, you'll be updating the affected recipient so that mail is forwarded to the onMicrosoft.com "fallback" email alias address.

1. Go to the Email recipients control in your SpamHero control panel (https://www.spamhero.com/cp/settings/recipients)
   Or navigate there by clicking Settings > Email recipients in the SpamHero control panel.
2. Find and click on the recipient user that is having trouble receiving messages. example

   If needed, use the search box on the upper-right side:

   
   

3. In the Forwarding addresses field, enter the recipient's onmicrosoft.com fallback alias address, as found in the Microsoft 365 admin center
   • Skip the "Also deliver to original recipient" checkbox (otherwise, the sender will receive a bounce message)
   • Click on the I understand... checkbox
   • Click on the Save button
   
   

Allow a minute for the back end to process the update before re-queuing the message for delivery.

Re-queue failed deliveries

You can re-queue messages that failed to deliver because of this error. Keep in mind that because the message was looping, several duplicate copies will appear in your message viewer. Only one copy of each message needs to be released. It's also advisable to re-queue a single message first to confirm that the looping issue has been resolved.

1. Navigate to your SpamHero quarantine and find the message that failed to deliver.
2. Hover over the message and click on the Deliver link.

The message should be delivered to your in-box within a minute. Other messages from senders that are affected by the Microsoft 365 mail loop bug should deliver without further changes.