Q: mail application does not open on Macbook Air after Yosemite 10.10.3 install

When I tried to open my mail application on my Macbook Air a window appeared on top of the Mail application asking me to "Choose an email account to add".

Since I already had email accounts added earlier (Ive been using this mac for over 3 years) I clicked on Cancel which led to the application closing.

Then when I tried to add an e-mail account it would say "that email account is already added"

So, in summary, the mail application is not opening.