How do I allow someone else to manage my mail and calendar in Outlook?

Similar to having an assistant that helps you manage your incoming paper mail, another person, known as a delegate, can receive and respond to email messages and meeting requests and responses on your behalf. You can also grant the delegate additional permissions to read, create, or change items in your Office 365 mailbox.

Delegate Access goes beyond just sharing access to your folders or calendar. Delegates are granted additional permissions, such as creating email messages or responding to meeting requests on your behalf. See Manage another person's mail and calendar items to learn how delegates can perform these tasks.

For more information and detailed instructions, please review this Microsoft article:

Allow someone else to manage your mail and calendar