Hello,
In answer to your questions:
1. Yes I am using 15.6
2. To clarify, the other accounts take the form david@companyB.com, david@companyC.com as well as some shared ones such as finance@companyA.com, support@companyB.com were all setup via our Office365 Exchange console.
The first two are setup as Security Groups with my primary account as my primary email address.
The others are setup as Shared Mailboxes.
With Windows Outlook, all the Shared Mailboxes appear automatically in my list of mailboxes and I can send as. For the two Security Group accounts, I can reply to email to those accounts by having it sent from the respective security group.
With OS-X Outlook, I had to go into Accounts, Advanced, Delegates and manually add the various security group / shared mailbox email "accounts" under the "People I am a delegate for" box. Having done that when I try to send from david@companyB.com by changing the entry in the From: box the email is going straight into my Drafts folder after I click on Send with no explanation or feedback at all. As I said before, if I use Outlook for Windows, no problem.
My basic question remains: Is Outlook 2015 (15.6) for OS-X supposed to behave the same as Outlook for Windows with respect to Security Group or Shared Mailboxes? If not is this documented somewhere? The answer to this is either Yes or No and a pointer to the relevant on-line documentation. Note I have the same question regarding the newly released IOS Outlook client.
I am happy to un-install Outlook2015 for OS-X and re-install if that is necessary, assuming asI said it should work.