Thank you for the question. Here are a couple articles that you may find helpful with this issue.
This is an easy fix. i called apple and they fixed it. Go to FINDER, then the GO tab and click the OPTION near command on keyboard and a link LIBRARY will pop up in list and then click LIBRARY, scroll to find KEYCHAIN and put it on your desktop and restart computer. You only have to log in one time and then delete keychain off desktop. something happened w maverick and it corrupted the file.
I tried this & it didn't work. I have only been having the problem since this morning. My computer was "asleep" overnight, not shut down. I have since tried re-entering my Mac Mail password, restarting, closing out of mail & reopening, etc. None of these things have worked. I did not do any recent software upgrade, either.