Not receiving e-mails from some senders.

Not receiving e-mails from some senders.

Hi.  We have been using Zoho for a few weeks now.  So far, we have known of two people who tried to e-mail us, but we never received them on the Zoho mail (using our domain).  However, one of them had sent the e-mail to my personal Gmail account, and I got it.

Any insights into what is going on and what we can do to fix it?

THANKS!