I was also assuming the problem was in Mail, but if you go to Comcast Cust (cussed?) Service page, you'll find multiple report from customers (cussed-tomers?) reporting problems accessing Comcast mail servers for the last couple of days. (I have also been having trouble downloading mail) I had actually deleted all traces of Mail, reinstalled and copied backups of my old Mail folders before checking at the Comcast site. Ah well, Comcast explains that they are aware of problems with their servers for the last couple of days, (and will be better soon) and now my Mail is working as always
