We upgraded our office from Outlook 2010 to Outlook 2013 a little while back. Back when we had Outlook 2010 users were able to send emails from whichever inbox they where in. For example: they all have their personal inbox and two shared inboxes. If they clicked on the inbox of shared inbox 1 and then clicked "new email" the "from" field would be filled with shared inbox 1's email. Then if they clicked on the inbox of shared inbox 2 and click "new email" the "from" field would be populated with shared inbox 2's email. Now for some reason no matter what inbox they are in the "from" field is always populated with their default personal email.
We have a couple users still on Outlook 2010 and recently we had a problem with them and had to reinstall Outlook 2010. They have also lost this feature as well.
This tells me that there is a setting in Outlook somewhere that I can't find that sets this feature.
Oh and all personal/shared emails are company emails hosted on exchange servers.