While I understand why people are doing this, I believe it's not the right way to deal with the problem - basically we're handing over e-mail (as a service) to a few big corps. Each time I have this problem I go through the long and painstaking process of whitelisting the IP and fight to make it work. Usually having it work with Gmail, Yahoo and Microsoft is enough - many smaller orgs don't use balcklisting by default because they have enough problems with mail deliverability already.