return-path overrides from prematurely

related to changes made in #CRM-5946

in the above issue we set $from to be the value of the return-path header for the purpose of verp processing (around line 275 of packages/Mail/smtp.php).
but as a result, when we do our "valid FROM" check in line 291, we are using the verp'd bounce address rather than the actual from address. that can be a problem if the bounce account does not reside on the local server as the server may reject it.

I think we should reorder the code so that the original from email is used for that check, and then is set to the return-path value afterward.