My problem is the following:

My company recently changed to Zoho, because it was recommended to us as the best email service on the net. But ... we have encountered some random problems on our journey.

For example; Some incoming mails are not arriving at all (not even on SPAM folder). I have tried everything i found on the forums (i added the address on the white list, i applied a filter to mark it as urgent, etc) 

If they carbon copy me to any hotmail or gmail adress, it shows up immediately.

Whats the problem? 

Thank you!