I updated to Yosemite about a week ago, and suddenly this morning my Verizon email acct started asking for my password. Did not recognize when typed in. Did all the normal things, quit started email, restarted Mac, recreated acct. nothing. Called Verizon, but could not help me troubleshoot mail app. Chatted with Apple. To make a VERY long story short,a eft FOUR hours and speaking with 3rd tier email support at Apple...I had been given the wrong instructions to KEEP yahoo in the incoming and outgoing servers (even though I don't use yahoo) AND the wrong outgoing port. Auto config for email does not work as it once did for my Verizon acct...you have to do it manually. I STILL do not have my email from before Oct 23 on my Mac, but my phone and iPad are fine. Obviously a Yosemite glitch AND a Verizon issue. Apple's support was MUCH more helpful than Verizon's that's for sure...but 4 hours to correct! What a headache!