Follow the below steps to configure outbound routing for Office 365. 

Step One

1.1 Login to the Office 365 Administration Console.

1.2 Select the Admin. 

1.3 Click Admin centers and click Exchange.

Step Two

2.1 Select the Mail Flow | Connectors menu item and create a new Connector.

Step Three

Complete the relay.p8mail.io Connector - Select Your Mail Flow Scenario dialog as follows:

3.1 Make sure you select From: Office 365 and To: Partner Organization.

3.2 Name the connector relay.p8mail.io

Step Four

4.1 Add * as a senders domain.

Step Five

5.1 Route email through relay.p8mail.io 

Step Six

6.1 Select `Always use Transport Layer Security (TTL)` and `Issued by a trusted certificate authority (CA)`

Step Seven

7.1 Add an email address for validation.

7.2 Once you have added an email address, click on Validate.

Step Eight

6.2 Confirm settings and save.

Step Nine

Updating the SPF Record for your Domain(s)

Your organization should already have an SPF record for the domain(s) registered with Office 365. This SPF TXT record must be updated in the DNS zone for the relevant domain to reflect the following TXT record:

• Remove: "v=spf1 include:spf.protection.outlook.com –all"
• Replace with  "v=spf1 include:_spf.p8mail.io -all"

If you would like to use both 0365 and us then just add `include:_spf.p8mail.io` to the existing SPF record