Hi Rafael,
I think we have all the same problem. Here in France, a standard is 100 mails per hour. What I have done is a 'slave app' sending one email each 40 seconds. The main app fills a table with all datas needed to send the email : @, text, files to join, etc. It tooks a very short time and the user has control again. And each 40s, the slave look in the table if there is a mail to send and if yes send it. I have not done a "test if email is well sent" (bad adress, ...), they are received in the standard 'outlook'.
HTH The Foxil