Mail certificate handling bug

I received email from Intuit- legitimate mail- and got a certificate error message. Following prompts I asked that  the certificate be installed. Apparently something failed and the original pop-up window came back and could not be closed. I had to shut down Mail using Task Manager. Now any message from Intuit seems to prompt the same  problem. In order to remove the offending message I removed temporarily all the other messages following it in the Inbox but leaving two unwanted, unrelated, messages before it, and used multiple selection (shift click) to get a group of messages which I could send to Deleted items without directly clicking on the problem message. My problem is temporarily fixed, but I expect it to reappear if I get another message from Intuit. Is there a remedy? (I am using Windows Live mail, nee Outlook Express, with Windows 8.1 fully updated.)