Neemo's right. Our forum is primarily for folks using Windows and Linux computers; be we have some familiarity with Apple systems. It would be helpful to know which Mac, iPad, or iBook you and your recipient has along with which version of IOS each computer is running. Furthermore, are you or your recipient running Bootcamp or some sort of Virtual Machine on either of the Apple computers you have (sender/receiver machines) such as VMware, or VM-hyperbox, or similar?
Getting all the information about problems like this up front is like pulling teeth; it's really hard for us to guess all the parameters of your system without you telling us. We are not there and cannot physically see your equipment, settings, apps, configurations, etc.
Also, there is no such thing as
"Outlook 10" as you mention. There is W10 Native Mail (this looks like Outlook but is not a separate program that you pay Microsoft for to run (as Outlook 2010, part of Office 2010). It would be helpful to know exactly what your recipient has running on his computer.
Finally, if he is using W10 Native Mail (the built-in E-mail app in W10 that replaced Windows Live Mail (WLM)), and you are using Outlook 2010 or similar (older or newer version of Outlook such as Outlook 2007 or Outlook 2013/2016), I'd suggest you install one of these versions of Outlook on your recipient's computer. If it works, you've fixed it!
You should also be aware that W10 Native Mail, if that's what your friend is using, is a stripped down version of earlier Windows E-mail systems and has many features and capabilities removed from it (such as Delivery Reciepts, and Group Lists), and may be causing this particular problem.
We haven't done complete testing on the newest AU September Microsoft update's effect on all the different types of E-mail clients on the PC platform let alone on the Apple platform and all their devices, especially given the several different ways you can run virtual machine or emulation software on Apple computers with W10.

I personally don't have any of these, and therefore can't and don't spend the time to test these particular types of problems. Hopefully, some of our other Techs who do, can chime in and make some suggestions.
Best,
<<<BIGBEARJEDI>>>