Alternative to Alexanders comment ask your ISP to turn of SPF
Sender Policy Framework (SPF, http://spf.pobox.com) is a mechanism for preventing sender forgery in SMTP transaction, thus allowing domain owners control over who may send mail from their domain.
Than you are able to send email to their mail server without any authentication.
Sometimes when you use mail server IP address with the SMTP port like.
{mailserverIP}:25 it might work but that is not guaranteed.