How to list all mailboxes that an Exchange online user has access?

Connect to Exchange Online PowerShell

Run powershell with elevated privileges

1. Connect-ExchangeOnline

2. List all mailboxes to which a user has Send As permissions:

PS C:\> Get-Mailbox | Get-RecipientPermission -Trustee Katy

3. List all mailboxes to which a particular user has Full Access permissions:

PS C:\> Get-Mailbox | Get-MailboxPermission -User Katy

4. List all mailboxes to which a particular security principal has Send on behalf of permissions:

PS C:\> Get-Mailbox | ? {$_.GrantSendOnBehalfTo -match “katy”}