INVALID_ADDRESS, ERROR_CODE :550, ERROR_CODE :User wishes to be unknown to you. You are on dnsbl.sorbs.net (Currently Sending Spam See: http://www.sorbs.net/lookup.shtml?135.84.80.217)

INVALID_ADDRESS, ERROR_CODE :550, ERROR_CODE :User wishes to be unknown to you. You are on dnsbl.sorbs.net (Currently Sending Spam See: http://www.sorbs.net/lookup.shtml?135.84.80.217)

When I send mail from my Zoho mail account, i get a NDR message back containing the following:

INVALID_ADDRESS, ERROR_CODE :550, ERROR_CODE :User wishes to be unknown to you. You are on dnsbl.sorbs.net (Currently 
            Sending Spam See:  http://www.sorbs.net/lookup.shtml?135.84.80.217 ) 


Seems that your server(s) are on the sorbs list for sending spam? Can you fix it?