If I read correctly from documentation (http://msdn.microsoft.com/en-us/library/system.net.mail.smtpfailedrecipientsexception(v=vs.110).aspx) you should be able to read failed addresses from FailedRecipient property of Exception. To get proper type of exception, change your catch block to handle SmtpFailedRecipientsException kind of exceptions.
catch (SmtpFailedRecipientsException smtpSendException)
{
err.setErreur(smtpSendException.FailedRecipient...);
}