Folks having problems with eM Client using Comcast/Xfinity please be aware that they changed their SMTP port. Port 25 no longer works. Three ports are available to choose from; 995, 587,465.
I tried 995 with no luck, 587 worked for me. You may have to try each one until you find the one right for your computer.
I have seen questions about this and thought I would pass on the information.