Apple Mail keeps saying message can't be saved
I keep getting this message pop up: "Error: The message could not be saved." The only option is to click OK. I click OK and it comes up again, sometimes 10 times before it finally stops popping up. Despite the error pop-up, messages are still there; they don't disappear and when I'm done, I can send them. What is going on here?
iMac Line (2012 and Later)