I have been using Zoho Mail for some time now and everything works fine except that I am not getting emails from certain people such as my college professors. I am getting emails from all other people except my college professors.

Can you guys please tell me what are the steps to troubleshoot this problem? What could be wrong?

I even checked the spam section and there is nothing there.

By the way, my professors are using Gmail as their email provider/

Do I have to adjust any settings in my email?