Keep receiving ERROR_CODE :550, ERROR_CODE :"The mail server detected your message as spam and has prevented delivery.

Keep receiving ERROR_CODE :550, ERROR_CODE :"The mail server detected your message as spam and has prevented delivery.

Every time I try to send a messagge I get this ERROR_CODE :550, ERROR_CODE :"The mail server detected your message as spam and has prevented delivery.

¿What can I do?
Thanks.