Assign Full Access and Send As permissions on all mailboxes

Grant user@domain.com Full Access to all mailboxes:

Get-Mailbox | AddMailboxPermission -User user@domain.com -AccessRights FullAccess -AutoMapping $false

Grant user@domain.com Send As on all mailboxes:

Get-Recipient | Add-RecipientPermission -AccessRights SendAs -Trustee user@domain.com