How do I get Outlook to compose new emails from the default account and not ignore the actual default setting on a Mac (Office 365)?

Question

I run Office 365 on a Mac (Yosemite) I have two accounts in Outlook. I have set a default account. However, each time I compose a message, it overrides my default account and opts for whatever account is currently in view. I do not prefer that behavior.

I see how to do this on a PC:

How do I get Outlook to actually default new emails to a specific account and not ignore the default setting?

I am stuck at how to get around this "feature."

Will someone please point out the workaround for the Mac? I suspect it is editing these files:

Library - Group Containers - Outlook - Outlook 15 Profiles

Thank you so much.