I keep receiving a screen message saying that there is a new version of 'Start menu reviver'. Nevererdofit! No indication of the source of the message.
So please is it from MS, or perhaps a nasty?
It's not from Microsoft but from a software company called ReviverSoft
As you see it's basically designed for Windows 8 to restore the Start button
It's not clear to me if this nag screen that pops up actually means you have this programme installed or merely have picked up some adware to get you to install it
Instructions on uninstalling are given here
If Malwarebytes doesn't regard it as malware you are probably ok and I don't think your e-mail problems have anything to do with this
Have you tried safe mode scans with Malwarebytes and Kaspersky?