Cannot Log Into Comcast E-Mail from Apple Mail 9

A couple of days ago, my iMac (Mail) stopped connecting to Comcast E-Mail. I can log on through the Comcast website, but cannot do so using the Mail client on my iMac. I can also log on using Mail on my iPhone 6. I have checked and re-checked the settings; Comcast says that since I can connect with them through alternative means, the problem must be with Apple Mail. Help!