Problem receiving incoming mail

Problem receiving incoming mail

Hi there, our business Zoho mail account has been set up and working (for the most part) properly for the past four months. Today, all of a sudden, we are not consistently receiving incoming email. As one example: I sent a test email from my personal Gmail account to my Zoho mail account two hours ago. It has not yet been received 

This is a huge problem, as our clients rely on email to contact us. Please advise on how this is being corrected on your end, or if we can take any steps to correct it on ours.

Thank you for your assistance.
Laura