Configuring preferences and options in Outlook.com
I found the reason I no longer can send from my Gmail account under the name of an Outlook alias that I have connected to via SMTP:
“Microsoft has removed the ability to connect third-party accounts such as Gmail and sync existing connected accounts from within Outlook.com,” that is, from the online version of Outlook from your computer. I believe that is their unclear way of saying we cannot use SMTP from a Gmail account online from our computer to send through an Outlook alias — the very point of this thread. (If I'm wrong, let me know.)
The info says you might still be able to add the third party account to other Outlook versions, such as Outlook mobile, the desktop versions (clients) of Outlook for Windows or Mac, or by forwarding the e-mails to Outlook, then sending from Outlook, a very burdensome way to accomplish it.
The article is not dated, but it must have happened early this year:
They give no explanation of what they changed to do this. Without an explanation, it seems ludicrous to me to still allow that ability if you do it via a desktop client or mobile — where is the logic in that?