A quick look at the App Store will confirm the above comment from chscag. If you search for Mail it will list among the alternatives Yosemite, why, because the only way you can get Apple Mail is by downloading a whole operating system.
OS X Yosemite
MacBook Pro (15-inch, Late 2011)
Processor 2.2 GHz Intel Core i7
Memory 4 GB 1333 MHz DDR3
Startup Disk Macintosh HD
Graphics AMD Radeon HD 6750M 512MB
I upgraded to Yosemite and that was a mistake, nothing but problems that I didnot have before. Now I can't even open up an application without having to watch the spinning wheel. Even highlighting text is a problem because the cursor will not react and I have to wait for something to catch up. Mail was a major problem, I sent one email with about 35 photos in it. The total size of the email was 2.7 MB, but Mail kept making copies of it. At one time I have 53 copies of this one email in the recovery folder that I could not delete. One site suggested that I delete the V2 folder and resign in my email accounts. I tried that, but now I can not sign in as the window deletes what I have entered in the form when I press Next and when I press cancel the program quits. I was hoping reinstalling Mail.app would solve this problem...