I use this PowerShell script to read a CSV file enumerating the mail column. The mail column is a list of Office 365 email accounts in a hybrid Office 365 Exchange target tenant.
When generating the CV the user’s User Principal Name (UPN) usually matches their mail attribute. This can vary with more complicated configurations, or if a tenant has multiple domain names.
A PowerShell session must first be connected to Exchange online service. A quick guide is here: Connect to Exchange Online PowerShell