I did a mail merge to e-mail in Outlook 2016 on Windows 10. Before i did this i ensured the default account was set correctly, but instead the e-mail went from the club’s gmail account and not our own domain.
Turns out that the default account in Outlook is frequently ignored and uses the address for the message store that currently has focus. Now annoyingly i use this every day at work, and i’m aware of the behaviour and actually like it. However, this is not so great for sending of mail merged e-mail.
On having a google around I came across this post http://www.slipstick.com/outlook/outlook-2010/multiple-accounts-and-the-default-account/ which details how to get this fixed. I’ve included the registry links after the break should this site go down.