I had the same problem. I was using QMail on the server, and all attempts to send failed. I tried the identical complete site and database on a server with just sendmail, and it worked.

I stopped qmail and reinstalled sendmail on the first server, and it worked. I haven't tried any other MTAs or tried PHP Mail or SMTP yet.

I also noticed that the upgrade contains the phpmailer.lang-en.php file in the language folder, but it isn't transferred to {url}/class/mail/phpmailer. There is a README file that tells you how to set the language.