When sending email through Office 365, your local IP address will be injected into the message as an extra mail header. If you use Office 365’s webmail interface to hide your IP address, you are not hiding anything. Microsoft removed the x-originating-ip field in 2013 from Hotmail to offer their users more security and privacy. This is especially useful for finding the location of a sender in the event an account has been hacked. For Office 365 admins who do not wish to continue using this header, you can create a new rule in the Exchange admin center.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-office-365-webmail-exposes-users-ip-address-in-emails/