Is there a reason why you haven't updated from OS X 10.8? Usually the best fix is to run the latest update - the new Mail in Yosemite may well solve your problem.
Alternatively, you could try setting up your account in Mail again. Based on the fact that it is working for your wife it would seem that there is something in the settings that might be rectified if you went through the whole set up again.
But I'd definitely recommend updating, I think that will solve the issue.
But make sure you back up first!
Read our guide to updating to Yosemite over on Macworld.co.uk : click here