não estou conseguindo mandar e-mail para alguns lugares, alguém sabe mim dizer pq isso acontece, e estou recebendo a seguinte mensagem

Hi. This is the qmail-send program at systemnet.com.br.
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up. Sorry it didn't work out.

<[email protected]>: does not like recipient.
Remote host said: 554 Service unavailable; Client host [] blocked using bl.spamcop.net; Blocked - see http://www.spamcop.net/bl.shtml?
Giving up on