I'd like to know the same thing! It started very recently. I thought I'd hit "delete" by accident, but this has happened several times now and I haven't changed anything or gotten any alerts to a change from Xfinity. VERYannoying!
I just experienced the same...responded to an email, but there is no record of the response. If I create an original email, it will show up in sent, but not responses. Anyone know how to adjust the app so all sent messages are documented?