Sending emails from comcast.net. I can receive emails to this account, but not send. What to do?

I have two email addresses on my Mac, my iPad, and my iPhone. My gmail account is my primary email on all three devices, but I also have a second one through Comcast set up on all three devices. I have no problem sending and/or receiving emails from my gmail, and I can receive emails from my Comcast account, but I can't send anything from the Comcast account. How to fix??