Allowing someone else to manage your mail and calendar - Exchange Online

Setting up your mailbox so that someone else can manage your email, calendar etc.. is called "Delegate Access"

Microsoft has an excellent page on how to set up Outlook for "Delegate Access", so that someone else can manage your email or calendar , see https://support.office.com/en-ca/article/Allow-someone-else-to-manage-your-mail-and-calendar-9684b670-7588-4eea-8717-9e5799047540.