We have had our email and domain hosted with Zoho since purchase and for the most part love it. However, there are several parties we try to email where we don't even make it into the spam folder and we're being blocked completely. I don't understand why this is happening. Even if I send the email from my personal Gmail with my Zoho account attached in the CC and they reply to all it does not add the email to their safe/white list. I never had this issue when I created our own email domain through Google years ago.

We are trying to run a business and make it more professional than using a Gmail domain, but if our messages can't go through then that's worse than looking unprofessional.

How can we fix it so our emails aren't going to spam?