Windows Server PHP mail being blocked by Comcast/AOL

I have a windows server running an application which sends mail via PHP. Some of the people who should be recieving the emails are complaining they are not ge开发者_如何学编程tting them. I think they are using Comcast or AOL. What can I do to my SMTP settings and/or DNS to allow these to go through?

Investigate SPF (Sender Protection Framework) to increase the authenticity of your emails.