The following steps with allow the end user to send an email via a shared inbox.
Step 1 - Open the Outlook Desktop App
Step 2 - Select File in the upper left corner.
Step 3 - Select Account Settings.
Step 4 - Click on Manage Profiles
Step 5 - Click on Show Profile
Step 6 - Click on Add.
Step 7 - Type in a Profile name and Click OK.
Step 8 - In the Auto Account Setup box > the email will autofill to your atu email address > You will need to change Email Address to the shared email inbox address > Don't enter a password > Click Next
Step 9 - Auto discovery with take place and you will click Allow.
Step 10 - You will be prompted to sign into Microsoft, in this box change email here to your ATU username account that should have access to the shared mailbox account.
Step 11 - Click on Next and this will add the mailbox as profile for you to choose from.
Step 12 - Make sure "Prompt for a Profile to be used" is selected in the Mail Profile window > Then Click OK.
Once you do this Outlook will want to restart your Outlook program. It will then allow you to choose either the shared inbox profile or your ATU username profile. When you launch into the shared email profile you will be allowed to send mass emails to a larger group.