Error 550.5.1.1 for a mailbox of my domain

Error 550.5.1.1 for a mailbox of my domain

Hi

When i try to write to daniel.houwaert@dho-online, the mail server of Zoho give me : dany.houwaert@dho-online.com INVALID_ADDRESS, ERROR_CODE :550, ERROR_CODE :5.1.1 < dany.houwaert@dho-online.com>: Recipient address rejected: User unknown

But when a write to dimitri.houwaert@dho-online.com or a other email address it's work !!!
And of course the user and the mailbox exist in my control panel !

Can you help me

Dimitri Houwaert
Owner of dho-online.com