Urgent Help Needed, Some Mails are not being able to received

Urgent Help Needed, Some Mails are not being able to received

Hi all,

recently a couple of days back,
i couldn't received some of my customer emails.
while i can received some.

this is causing some seriousness in my daily operations for my business,
would some zoho staff kindly step forward and explain what is going on here?

please help