email disappearing

Click on email and often it disappears without dopening. IF I glimpsed who sent it, I can retrieve it by doing a Search for the sender's name. Otherwise, it is gone forever. By disappear, I mean it vanishes; does not go to any place on computer that I can find it. IF I'm able to open it, then it will disappear. This has been happening several weeks. One day about thirty missing messages reappeared all at once.  I have tried working with Comcast tec h support and was told to read a pamphlet. Geek Squad and Microsoft tech support have all examined computer and say this is an xfinity problem. I do business on my computer and this loss of email messages has serious consequences.