Mental note for me, as I am studying for MCSA 70-347.
# Send As Permissions for jblue, allowing jgreen to Send As. No indication the message was send by the delegate.
Add-RecipientPermission "jblue" -AccessRights SendAs -Trustee "jgreen"
# Full Access for jblue, allowing jgreen to have Full Access to the jblue mailbox. Please note messages cannot be send by the delegate.
Add-MailboxPermission -Identity "jblue" -User "jgreen" -AccessRights FullAccess -InheritanceType All
# Send Behalf, allows the delegate to send on behalf of the orignal user but does not hide their identity.
Set-Mailbox -Identity "jblue" -GrantSendOnBehalfTo