Welcome to managing all mailboxes and mail flow using Microsoft 365. Microsoft recommends that organizations use hosted mail flow in Microsoft 365 mainly because it's the easiest to set up. Configuring mail flow in this manner allows Microsoft 365 to manage all mailboxes and mail filtering. A typical use case for this type of mail flow architecture would be a new Microsoft 365 customer that wants to host all mailboxes in Microsoft 365 while leveraging all the filtering solutions that come with Microsoft 365. 

This hosted mail flow option is also a good one for again, new Microsoft 365 customers, but those that have an existing email service but are planning to immediately migrate all of their mailboxes to Microsoft 365 in order to take advantage of those filtering solutions that are included with the Microsoft 365 platform. The image on your screen shows what this type of architecture would look like. Microsoft recommends using the Microsoft 365 setup wizard to set up hosted mail flow. This wizard is accessed by clicking 'Setup' in the Microsoft 365 Admin Center. The wizard will walk you through the steps that are necessary for setting up hosted mail flow. You'll be prompted to add your custom domains in Microsoft 365 and you'll need to verify ownership of those domains. 

You'll then either create your user mailboxes in Exchange Online if you have no mailboxes to migrate or you'll move all of your user mailboxes to Microsoft 365 if you're indeed migrating from an existing solution. You'll also need to update the DNS records for the email domains that you added the Microsoft 365 including the MX record, which controls mail flow from the Internet to your domain. You should also update your SPF record in DNS as well. This record identifies the service as a valid sender for a particular email domain. So, since Microsoft 365 will be sending all your email messages, in this scenario, you need to list only Microsoft 365 as a valid sender for your domain. For a step by step instructions, you can visit the URL that you see on your screen.