Mimecast Administration Console - Inbound Delivery Routing For Microsoft 365

This guide describes how to route messages from us to Microsoft 365. It must be completed once your Mimecast account has been configured.

Overview

Mimecast works with Microsoft to enhance messaging security. As such, to ensure inbound messages are delivered and not incorrectly marked as spam, the Mimecast service IP ranges will need to be added to a Connection Filtering policy.

1. Complete the Connection Filtering process.
2. Continue with the Determining Host Name section.

Connection Filtering

For full details, see the Microsoft article here on how this functions and how to configure this feature. An outline of the steps required is also provided below.

To do this:

1. In the Defender Portal, expand Email & Collaboration in the sidebar on the left.
2. Navigate to Policies & Rules | Threat Policies | Anti-Spam in the Policies section.
3. Select the Connection Filter Policy in the policy details section to modify the policy settings.
4. Using the Edit connection filter policy option, configure the following setting:

Always allow messages from the following IP addresses or address range: This setting is the IP Allow List. Add the Mimecast IP Ranges. See the Data Centers & URLs page for further details.

5. Click on Turn on safe list.
6. Click on Save.

Microsoft Defender

Safe Links

When creating an inbound connector, Mimecast recommends disabling Microsoft Defender safe links as this can conflict with Mimecast URL protection; see the Safe Links in Microsoft Defender for Office 365 page for full details.

This is supported by Microsoft. Should any issues arise, please contact them for assistance.

Determining the Host Name

Our delivery routes are configured to deliver all inbound messages to a specified hostname, which, in this case, is the MX record for the Microsoft 365 account. To deliver messages from us to your Microsoft 365 service, you must determine your hostname:

1. Log in to the Microsoft 365 Admin Center.
2. Click on either of the following:

• • • Domains menu item.
    • Setup | Domains menu item.

3. Click on the Domain Settings tab for the domain you want to configure.
4. Note the MX value.

Configure a Delivery Routing policy to complete your inbound routing.

See Also...

• • • Data Centers & URLs
    • Connect Process - Office 365 mail lockdown
    • Microsoft Article - Enhanced Filtering for Connectors in Exchange Online
    • Microsoft Article - Connection Filtering
    • Microsoft Article - Using a 3rd Party Cloud Service