An unknown error has occurred Windows Live Mail

I have been using this Windows Live Mail for a few years on my Windows 7Pro 64bit without any problems. Just recently in the past few days I get an error window when I try to delete an email from the inbox. The window just pops-up on the screen and says, "An unknown error has occurred", and Windows Live Mail will not delete that email. If I continue with other emails I can delete most of them without any problems. I can't see a pattern to which emails are chosen to give the error window. The emails that Windows Live Mail will not let me delete seem to be chosen randomly. I have made no recent changes to my computer. I regularly run CCleaner, Windows Essentials, and Malwarebytes and I haven't had any viruses etc. I don't open suspicious emails. I don't tweet and I don't use facebook. Any ideas what may be going on and how to stop it?  My Computer Build: i7-4930 Intel 6 core, Corsair 950W power supply, ASUS motherboard, EVGA 780 graphics card, 64GB RAM, Samsung 500GB SSD for system drive and 4 HDD for Music, Photos etc. No other problems known with Windows 7 or computer, and no recent software added, and no other changes to computer. Help will be appreciated.