Customers that are using Email Security with Office 365, might face Hop count issues when trying to send emails from their internal email addresses inside their organization (from a user inside the organization to another user inside the same organization) or to another organization that is using Office 365 (and it is placed in the same forest as the Email Security servers).
Above: Example screenshot of Error Code 1554 5.4.14 Hop count exceeded
Reported error: 550 5.0.350 Remote server returned an error -> 554 delivery error: 5.4.14 Hop count exceeded - possible mail loop ATTR1
Disabling the Heimdal-related inbound connector
This loop starts happening when you are using an Inbound connector that specifies Heimdal's Email Security IP Addresses.
Disabling such a connector should fix the loop issue that starts happening when you are trying to send out an email from your organization to a recipient that is also using Email Security.
Adding a pattern in the message header to disregard the connector
Another solution to solving this behavior would be to configure the Email Security Outbound transport rule to add a pattern in the message header and to disregard the connector when that pattern is found in the message header.
To configure this rule, follow the steps below:
2. Click Mail Flow > Rules.
3. Find the Heimdal Email Security Transport Rule (that manages the outbound flow) and edit the rule conditions by setting a Modify the message properties (set a message header) condition and an except condition - The message headers (matches these text patterns) with the X-HeimdalSecurity-Check pattern and Y value.
Besides routing the email (from Inside the organization) through the Heimdal Email Security connector/smarthost, we recommend settings the message header with the X-HeimdalSecurity-Check pattern and Y as value and except sending the email through the Heimdal Email Security connector/smarthost when this pattern is discovered in the message header so that it prevents the email loop occurrence.
4. Save the Heimdal Email Security Transport Rule and you're all done.