Welcome to Justanswer.com, my name is Dustin.I'm sorry to hear about the issue you are having, I will do my best to help you resolve it.
To start with, I need to know the version of mac osx you have on your machine, you can check as follows.Click the apple sign at the top left of your screen, then click about this mac. And then where it says version, send that to me as a reply.
Also, how do you normally access your mail.. Through your web browser, or through the mac mail stamp icon, or do you use a program like outlook 2011 or thunderbird?