Threatening calls and Email even when payment is made

I made payment on my The Fix account on the 10th August. However I keep receiving threatening calls, emails and sms's from The Fochini group. I have sent numerous email with the proof of payment which keep bouncing back with the error 554 Email rejected due to security policies. I have contacted you arrangements department and collections department , but still no joy. I was advised to send my POP to *** which is still bouncing back. What is the problem??