I have no idea what resolved it!! It just suddenly worked even though I did nothing but keep trying to send a test message to myself. It had also happened on my PC ,but that cleared up more quickly then it did with the app.
You can try deleting the app and then reinstalling it and see if that helps.
Good Luck . Would be nice if Comcast had made some sort of reply in this forum.
This is around the time they were planning to make changes to settings within email applications so maybe that has something to do with it.
Sorry I can't be more helpful.