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Keep Microsoft 365 and add PostPike

Microsoft calls it an internal relay domain, with a connector.

Let unknown addresses through

  1. In the Exchange admin center, go to Mail flow, then Accepted domains, and select your domain.
  2. Under This accepted domain is, choose Internal relay, and select Save.

Send them to PostPike

  1. Go to Mail flow, then Connectors, and select Add a connector.
  2. Under Connection from, choose Office 365. Under Connection to, choose Your organization's email server. Select Next and give it a name.
  3. Choose to use it only when email messages are sent to these domains, and add your domain.
  4. For the smart host, enter mx1.postpike.net and select the plus sign. Keep Always use Transport Layer Security (TLS) on.
  5. For the validation email, enter one of your PostPike mailboxes on this domain, select the plus sign, then Validate, and create the connector.
Then select Test on your domain's page.

From Microsoft: Manage accepted domains in Exchange Online, Microsoft: Set up connectors to route mail. Menus change: if something here doesn't match what you see, tell us.