I missed this post previously.
The first problem is that smtp provider is "spammy" and gets flagged for having lots of spam on its system. The second problem is their mail server doesn't seem "well configured" which is driving up the spam scores of messages coming from that isp. I've had similar problems with clients in China sending mail from the free email providers getting all their messages stopped cold as "high spam". Annoying and there is very little I can do about the ISPs reputation.
So, what do you do instead? If you know the addresses ahead of time coming from that isp, then you can add a negative spam score for those addresses/domains. Thus, if the ISP gets a score of +5 because they are assholes, then you can have spamassassin add a -5 score for
someuser@somedomain.com who uses that isp, thus letting their mail come in.
Were you able to solve this problem in the end?