Q: Microsoft outlook using different email address rather than default

I work for an IT Company for businesses.

Got a user of a MAC at one of the offices we work for and he is using Microsoft Office 2011.

He has 2 mail exchange mail accounts set up on his PC.

The issue that he has come across as that when he is in the "joint" inbox that shows all of the mail in. the user has one email set as default but it sends from the secondary when in the joint inbox.

Is there anything that can be done to prevent this happening or am I right in saying if he has a highlighted email from a certain account and opens a new email it opens with the mail address that the selected email has been sent to. Only way to change this would then to either be in the correct mail box (set as default) or change it manually from the send as option?