Unable to send/recieve emails.

Unable to send/recieve emails.

Hi,
We are using zoho mail since a month or so. Everything is working fine till now.
All of a sudden one of the user is unable to send/receive any email. This happened since morning.
There is no error. Just the emails are not going to the recepient nor is he able to receive any emails.Others are able to send/receive emails.

Could you pls help resolve...

Thanks
Ravi