HERE'S THE DEFINITIVE SOLUTION
Hello everyone, so I too have had this problem ever since I made the move to Mac, 12 months ago. Firstly with Outlook 2011 which often resulted in a complete mismatch of fonts on the recipients mail client, then more recently after I dumped Outlook in favour of Mail (after looking at all the other popular mail apps around which just didn't do the job I wanted). Mail doesn't have all the functionality that Outlook has, but it is the best alternative and the font results are nothing like as bad as I found with Outlook. But they're not good either.
The solution is simple. Universal Mailer. Several of the posts above recommend it, so I thought I'd give it a go last night. I downloaded and installed in on my Macbook Pro and within a few minutes of a tiny bit of trial and error playing with the font configuration, the problem was solved. Today I've done the same on my iMac and problem solved. All my emails are now being displayed on the recipients email client (usually Outlook) in my desired font (Calibri 12) and my signature block is consistent as well. Everything else I've tested and it's all working.
Universal Mailer works with Mavericks, Yosemite and El Capitan (I've tested it with Yosemite as I'm holding back upgrading at the moment), and it's free. Donations to the developers are optional, but I'm certainly going to make one - one major ongoing hassle solved. Fantastic.