"Address not found" on a valid address

"Address not found" on a valid address

Hi,

Masking the username and domain I had the error message below

The response from the remote server was (message sent from gmail to zoho):

550 5.1.1 < xxx@yyy.com> User unknown

Final-Recipient: rfc822; xxx@yyy.com
Action: failed
Status: 5.1.1
Remote-MTA: dns; mx2.zoho.com. (74.201.154.201, the server for the domain yyy.com.)
Diagnostic-Code: smtp; 550 5.1.1 < xxx@yyy.com> User unknown

the email exists, the domain too. The user fetch his mail on pop3 at zoho. Note we are using the free service. Anyone else with that problem?

Thanks