List all mailboxes to which a particular user has access

Get-Mailbox -resultsize unlimited | Get-MailboxPermission -User username@domain.com

Get-Mailbox -resultsize unlimited | Get-RecipientPermission -Trustee username@domain.com

Get-Mailbox -resultsize unlimited | ? {$_.GrantSendOnBehalfTo -match "user@principal.name"}

Source:

https://www.michev.info/Blog/Post/1516/quickly-list-all-mailboxes-to-which-a-particular-user-has-access