Errort 550 - Sender address is invalid [route]

Errort 550 - Sender address is invalid [route]

Hi,

Just started getting this error, but only when sending to one email address...

Error, ERROR_CODE :550, ERROR_CODE :"Sender address is invalid [route]: " Received:from mail.zoho.com by mx.zohomail.com with SMTP id 1438734787 222707.4528 710224439; Tue, 4 Aug 2015 17:33:07 -0700 (PDT)

i've removed the to and my email address

cheers,
simon