Cox SMTP server on blacklist

I have been unable to send an email from my iMac using the iCloud smtp server for a month or so. Today I found time to troubleshoot with Apple support. Cox is my isp and I can send from iCloud if I log onto the website but not directly from my iMac. With the tech support agent, we have eliminated every possible problem on my computer or on their server to be the cause of this. The only thing left is that it is a systemic problem with Cox Cable. The tech agent also mentioned that every user who was having this problem, and there are quite a few, also had Cox Cable as their internet service provider.  When I call tech support at Cox, they tell me they cannot fix it unless I pay for the upgraded support. I tell them that it is a systemic problem on their end and that if subscribing to the support upgrade and troubleshooting with them did fix the problem . .  . . It would mean that a class action law suit would be warranted. Could this blacklist issue be the problem?