While testing to send multiple emails, I successfully sent about 100 emails (with a 20KB pdf attachment), to the same email-address (my own), and they were all received. But on next attempt, my cPanel account was blocked, due to a "brute force attempt".

Are there any special precautions I need to take when sending bulk emails? I simply looped through below code without pause for each email. What type of alert could that give on the email server, and how should I avoid it?

  client = New SmtpClient(smtp, Convert.ToInt32(port))
  AddHandler client.SendCompleted, AddressOf OnAsyncSendComplete
  client.Credentials = New System.Net.NetworkCredential(usn, psw)
  client.SendAsync(mail, token) 

Should I wait for SendComplete event for each email before sending the next?