Once a domain is added and verified into Office 365 there is a default domain appended to every tenant. You can view your tenant setup by going to Setup and Domains within the admin portal. Typically, when you do the registration most admins will configure the tenant with the company name. For example, in domains, you may see “companyname.onmicrosoft.com

Using MXtoolbox.com, search for your domains first, then we will search for the default onmicrosoft.com domain but adding the company name in front.

This will show us that mail are all using Office 365 mail servers on Microsoft.