If you send email attachments, your data is already on someone else's system.
Your email is housed on someone else's system until you retrieve it. And even then it is often backed up on the mail server.
And even if you only transfer your data via floppy disk, you can still be at risk.
If your OS is provided by Microsoft, you don't know what they are doing with your data. With some technical background you can use some sniffing tools to make an informed guess, but due to the closed nature of the OS you really don't know.
Microsoft has been busted before for having their OS "phone home" and send data about you and your PC back to their systems.
And that's when everything is working as advertised. If you're running a Windows OS you have about a one in three chance of having some sort of malware installed on your system. Certain kinds of malware searches for logins and passwords to banking sites. Other kinds do keyword searches on your documents and uploads anything interesting to the bot master servers.
As a former network security professional, I'm much more content to encrypt and upload data to one of Google's servers in the cloud that I would be to allow Microsoft access to any of my data anywhere for any reason.
Let me try to simplify my answer.
Nothing I send via email, except within the company I work for, contains any data that I’m concerned with.
I don’t concern myself with malware because of the steps I take to limit my exposure.
Physical security is the name of the game. Whether it’s my laptop, desktop or data (that will not be remotely stored on any 3d-party cloud) taking care of that is 90% of the battle.