There was a typo in my previous email. Where I said "I am able to send messages to other email ..." it should have said "gmail" rather than "email". So yes, I have tried other gmail addresses, and there is no problem getting through to them. The problem does still exist when I send to support@ccleaner.com. I try sending a test message every day or so to see if the problem is still there.
Do ccleaner have their own email server as the address is not itself a gmail or googlemail address? If so, then that is probably where the block is happening. That would fit in with the wording of the reject message,