In order to send messages from a shared email account, you will need to specify the account when creating the email from a shared email box.
PC
Create a new message
Select the Options tab
Select From in "Show Fields"
Select the From drop down
Select Other Email Address
Enter the shared email address to search
Select OK
If a shared account is found, it will be added as the sender in the From field
If a shared account is not found, you will be promoted to search from a list
For subsequent messages, your shared mailbox address will be an option in the From drop-down.
Outcome: You should now be able to indicate the intended sender.
Visit the Shared Mailboxes and Calendars: Microsoft Office 365 page to create a ticket