We just completed a case study with ReturnPath on Reaching the Inbox and Rendering Solutions and how to design so as not to encounter these issue pre-campaign.

eROI uses this data primarily to educate, consult and design. We like to help the troubled mailers become good ones and enjoy the increased response that comes from improved deliverability. Of course, if education fails, they can segregate their IPs so top-notch marketers don’t suffer because of other mailers misdeeds. (Which is why eROI has so many IPs and spend so much time monitoring, balancing and segmenting mailers on volume, reputation, list hygiene, vertical industry (say banking), and even by brand.)
eROI also uses Sender Score Campaign Preview to help their clients with content assessments. Content is another area where many ESPs typically have little control, so having a mechanism to help clients pinpoint where they need to make changes is crucial. But eROI is a hybrid ESP with full service and self service clients, agencies and other partners; due to this they can head off or test for issues prior to a campaign drop that they are creating, or test on behalf of a self serve client that requests it. Read more about how eROI uses the Sender Score suite to help its clients.
If you are struggling with deliverability issues, talk to eROI about how they address these problems. They help you assess your content? They segregate high reputation mailers from low reputation ones to protect their deliverability. You can’t expect your ESP to solve the problems that you are causing in your program, but you can expect eROI to help you diagnose and solve your problems so that you can increase your email success. Why else would we have so many great long term clients and even more each month selecting us as a partner.
http://www.eroi.com/eroi-email-marketing-q207-reaching-the-inbox.html