I understand very well your question as I got the same problem
I sent 139. emails grouped by 3. each 10 seconds. All but one have been sent But I receiver 40 mail delivery subsystem messages as mail rejected "Ofr 506. or 416. or 550 spam detected ???? "How can enter the right setting to avoid that catastrophe because as it is the logiciel is not matching my use (excuse me for my poor English as I am French)