Yes, internet access is required to install and activate all Microsoft 365 plans and to manage your subscription account. Internet connectivity is also required to access Microsoft 365 cloud productivity services, including email, conferencing, IT management, and other services. Microsoft 365 Business Basic and Microsoft 365 Business Standard plans include the desktop version of Office.
The Microsoft 365 apps that you install on your PC or Mac—such as Word, Excel, PowerPoint and Outlook—are available to you when you’re not online. With OneDrive in Microsoft 365, you also get file storage that you can access when you’re offline. Changes (made while you’re offline) will be synced to OneDrive and across the rest of your devices when you reconnect.
With Outlook, read emails already delivered to your inbox, or draft new emails and meeting requests. Your inbox and outbox will sync across the rest of your devices when you reconnect.