My 2011 i5 iMac is running 10.9.5 and using the included Mail app for managing 7 email accounts plus archives from a couple more. I have a fair number of rules to keep things sorted out and organized pretty well, and I delete the cruft and empty the trash regularly. I keep the inboxes under 2000 items total. Sometime last year, I started noticing email coming in with old timestamps. The "received" dates were randomly in the past: anywhere from a few days to a few months old, with no real pattern. They didn't come from the same sender, or to the same email address. Some days I'll get 1 or 2, other days I'll get 50+ emails that I'm sure I received already, and deleted. It seems this is an issue with Mail; I temporarily deactivated some of my accounts on Mail and activated them via MS Outlook, and the issue stopped for those accounts. I used the "rebuild" function in Mail and transferred all accounts back to Mail and the issue seemed to abate, temporarily. It is back now in full force. Any ideas what is wrong with Mail, and how to fix it? I'd rather not switch to Outlook, but ya gotta do what ya gotta do.