Locking Down Your Office 365 Inbound Email Flow

Document created by user.oxriBaJeN4 Employee on Jun 22, 2016Last modified by user.Yo2IBgvWqr on Jul 9, 2018
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We recommended that you lock down your inbound email flow in Office 365 to only allow mail from Mimecast IP addresses. This requires you to create a receive connector in Office 365.


To lock down your firewall:

  1. Log on to the Office 365 Exchange Admin Console.
  2. Click on the Mail flow menu item.
  3. Click on the Connectors link. Your connectors are displayed.
  4. Click on the + icon.
  5. Complete the Select Your Mail Flow Scenario dialog as follows:

    FromPartner organization
    ToOffice 365
    The text at the bottom of the wizard changes to, “Creating a connector is optional for this mail flow scenario. Create a connector only if you want to enhance security for the email messages sent between your partner organization or service provider and Office 365. You can create multiple connectors for this scenario, each applying to different partner organizations or service providers”
  6. Click the Next button.
  7. Change the connector's name to Mimecast to Office 365.
  8. Click the Next button.
  9. Select the Use the Sender's Domain option in the "How do you want to identify the partner organization?” dialog.
  10. Click the Next button.
  11. Click on the + icon to add the * as the domain.
  12. Click on the OK button.
  13. Click the Next button.
  14. Leave the Reject Email Messages if They Aren't Sent Over TLS option with the default value on the “What security restrictions do you want to apply?” dialog. Mimecast will send the message on to Office 365 with Opportunistic TLS.
  15. Select the Reject email messages if they aren't sent from within this IP address range option.
  16. Click on the + icon to add the Mimecast IP address ranges depending on your region. See the Mimecast Data Centers and URLs page for full details.
  17. Click the Next button.
  18. A summary page is displayed. Check this to ensure it has all the correct information.
  19. Click on the Save button.
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