550 5.1.1 Error - no emails coming through -- help!

550 5.1.1 Error - no emails coming through -- help!

I understand that this may be due to the fact that I don't have the accounts set up in Zoho. However, when I visit control panel, it shows my main account there (that too is bouncing back with a 550 5.1.1 for some reason). I have tried creating my user accounts but I keep getting the message "User already exists in your org." Any ideas? This is infuriating.