I have my own domain name and I had some issues with the delivery time. So, I started looking at the mail header to see where my mail was stuck and why. That is not the issue.. While messing around the header file I noticed that the mail header not only contains the device used for the mail but the version of the operating system on my device. Here is the data:
X-Mailer: iPhone Mail (13B143)
13B143 is the code of iOS version 9.1.
Why on earth should anyone know the exact version of my OS? Is there a way to prevent that? Is there an other application more discrete?