I've been using a homegrown solution for years. I've learned more than I ever wanted to about spam filters, having IPs greylisted, throttling delivery speeds e

I've been using a homegrown solution for years. I've learned more than I ever wanted to about spam filters, having IPs greylisted, throttling delivery speeds etc. I've decided it's not worth our time anymore to keep up with all the latest requirements at hotmail,yahoo,gmail,comcast etc. So we're in the final stages of switching to http://cakemail.com http://cakemail.com as our "back end." Their api has worked flawlessly and lets us control everything. So far we're happy with them. Another good solution (but expensive unless you get to really high volumes) is strongmail.