I am in charge of an account but I am not receiving some mails

I am in charge of an account but I am not receiving some mails

Hi, I started to use Zoho with my clientes, but last week we had some troubles with our mail. People did send a lot of emails confirming their assistance to one of our events, but we had some problems with some mails. Our clients claim they send us mail that never reach our Inbox, even thought they did send mail before with out any problem. Why would this be happening?