ir_mail_server ignores failed recipients from smtp.sendmail

When sending a mail through the ir_mail_server.send_mail() method with multiple recipients, no exception is raised when the smtp-server refuses an address if there is a least one address that passes. In that case, smtplib does not raise, but returns a dictionary for each failed recipient (see [1]).

Here is an example of such a return value:

   {'<email address hidden>': (554, '5.7.1 <email address hidden>: Recipient address rejected: Access denied')}

OpenERP does not check the return value of smtp.sendmail and the failed delivery is never registered.

[1] http://docs.python.org/2/library/smtplib.html#smtplib.SMTP.sendmail