Mail sending blocked: Client host blocked using dnsbl.sorbs.net

Mail sending blocked: Client host blocked using dnsbl.sorbs.net

I am unable to send e-mails to the domain @caixa.gov.br

After sending e-mails, I got the the following message:

heron.r.santos@caixa.gov.br  INVALID_ADDRESS, ERROR_CODE :554, ERROR_CODE :5.7.1 Service unavailable; Client host [135.84.80.216] blocked using dnsbl.sorbs.net; Currently Sending Spam See:  http://www.sorbs.net/lookup.shtml?135.84.80.216  

Please help.

Thank you.