Microsoft Exchange Online is the system where all the main actions of the users happen, and that is why it is a core part of the platform. It is where users have their inboxes, calendars, and so on, and where they access the services that they need for their daily tasks, either using a mobile, tablet, or other devices like a PC.

In Exchange Online, there are some limits to avoid behaviors that could damage your reputation. For example, it's only possible to send 30 emails per minute from a user's mailbox. If you need to send emails in bulk, for a marketing campaign, for example, it is advised that you use third-party services or use other systems with different infrastructures.

Here, you can see the view of a mailbox in Office 365, using Outlook Online: