Here is another thing that makes me think it is Microsoft (I hear what several of you are saying about it being all over the Apple OS). I never have this blue arrow in my Apple Mail. I have four different emails all in that one Apple Mail and use it constantly.
I just got off of an employer site where I was logged in (to Microsoft and Google Drive); I try my best to avoid Microsoft as they are simply going too far these days.
The blue arrow is now all over my email. It isn't only in Microsoft documents (I have a lot of personal ones as well), it is literally inside the OS. I'm pretty sure that at one point I (stupidly and in haste) gave permission to Microsoft to access my Safari. I do not use Outlook nor do I ever want to. So this is to throw confusion into the brew. Why is that strictly Microsoft-looking fat blue arrow in my pristine, efficient, and clean Apple Mail? I seriously doubt that it is Apple. It is something that has access to the Mac OS.