unable to send email to comcast and earthlink

Two of my friends who have comcast and earthlink email accounts respectively are not receiving my emails. I have tried emailing them from my primary .mac address as well as from a number of my .me aliases.

We have verified this by talking on the phone while sending/receiving emails.

Running Snow Leopard 10.6.8 on a Macbook Pro 2.4 GHz Intel Core i5. Using Mail application 4.6 (1085).