If you are using Office 365 as your mail hosting provider and you are trying to set up Email Security MX Records, it is possible that you might stumble upon the below error while trying to validate our MX Records:
Why is this happening?
By default, Microsoft's Exchange and Exchange Online Protection offers the spam filtering solution needed to scan your company's environment. Because of this, Exchange recommends and prefers its own MX Records with Priority 0 (example: 0 example-com.mail.protection.outlook.com TTL 1 Hour). When it comes to a 3rd Party spam filtering solution, Email Security acts like the 'man-in-the-middle' between the Internet and Microsoft Exchange:This means that the domain's DNS settings need to be configured with Heimdal's Email Security MX Records instead of Office 365 MX Records. The Office 365 MX Records are then configured in the Heimdal Dashboard so that Email Security can successfully deliver the filtered emails to Exchange. Since Office 365 prefers its own MX Records, this is why it does not validate any other MX Records. To good thing is that although the Email Security MX Records cannot be validated, the email flow works just fine and emails can be seen in the Heimdal Dashboard, under Email Security.