Problem sending email within host

I just recently transferred my company email account from Squirrel mail to Zoho mail to host our company email. I am currently able to send and receive email from outside providers (yahoo, hotmail, gmail, etc) but I am unable to send or receive emails from my coworkers with the same domain name and host on Zoho. This problem goes for all the email addresses that is attached to my account. Does anyone know the reason for this?

Thanks and much appreciated