I was finally able to get this to work on a Windows 7 computer... it was failing on my Windows 8 box.
Send as does not work smoothly... it is just as hard to send as it was to set it up (powershell, really?).
But to even "send as" you have to click on options, then pick "show from". There is no option to always show the from. Gmail has a simple drop down menu on From and you pick the person you want to send as. Also, in gmail, if you reply to a message sent to your "send as" email, that same email will be the "from" if you reply. In office 365, that is not true... if you reply, you have to manually add the send as again.
"Send as" is very difficult to setup and use. I sometimes wish Microsoft would focus on making one thing great... but we have office 365, outlook.com and hotmail.com... and now all of these email apps in Windows 8. None of them work properly for my needs (great if they work for you).
I'm stuck with gmail and I don't even like google.