In my Outlook 365 client I have:
my personal account (me@myworkplace.co.uk)
a shared mailbox (stuff@domain.com)
another shared mailbox (otherStuff@domain.com)
When I click 'New Mail' then the new message defaults to being 'From' whatever account I've got selected and when that message is sent it passes through the relevant Outbox and ends up in the relevant Sent Items.
As far as we can see my colleagues have the same set up, i.e. they have their personal account and the same shared mailboxes that I have and all the settings for these accounts look the same as mine and everybody, including me, have had the same reg. patches to 'delegate sent items' applied.
HOWEVER, when my colleagues click 'New Mail' the new message defaults to being 'From' their personal account and ends up in their personal Sent Items.
![HairOut :hairout:](./images/smilies/hairout.gif)
Ken