Yesterday I discovered I wasn't receiving all my email on Mail on my iMac. I knew they were missing because I saw them on my iPad and wanted to reply from my iMac. I also checked my account on my browser. I rebooted, checked accounts, re-entered my password, online status, etc., when finally today, it re-loaded. Now it's really messed up!!! The sender doesn't correspond with the emails!!!
I upgraded to Mountain Lion a few weeks ago and didn't notice anything weird, but maybe it's been happening longer than just this past week. I don't think so, though. Mail has always worked beautifully before.