(Sorry I just got to checking back here.)
Thanks... In reply to your first post, I rarely send more than 100 emails a day. I use Mailchimp to send newsletters and rarely have an issue with those. As for dedicated SMTP service, it seems a bit over the top to spend over a thousand dollars per year just to get email delivered to the half dozen people I know who insist on using TWC.
I'll look into your suggestion in the second post, but I'm not sure I agree with the last statement. Time Warner is the only receiving ISP I have this issue with. If the problem was totally with my ISP I would expect to have difficulty sending email to other addresses. I would also note that it took MONTHS to get TWC to even respond to me--even if only to blame it on my ISP. My ISP has a much better response rate and has always been able to resolve my issues. I'm willing to concur they way they are using rotating servers sounds like it could be part of the issue (when TWC finally did respond it became apparent they couldn't help me because of that), but a similar issue a while back with AOL has been resolved, so a solution must be possible when providers work together.
I can't, unfortunately, give up on this. I'll post any progress I make. Ironically, over the last couple of days an occasional email to those TWC folks seems to get through, but it's not at all dependable.