Calendar invites interplay with Exchange/Pop accounts

Not sure if this is in the right spot or not, but I have a quick question.

I have a user with an in-house Exchange server and Outlook 2010.  In that Outlook we also have a POP account setup for the user, so there are 2 accounts total, Exchange and POP.

Whenever someone sends a calendar invite to the POP account, and the user accepts, the calendar event shows up on the Exchange calendar, WHICH IS FINE.  The problem occurs in the reply.  When the user accepts the invite and chooses "send the response now" the reply is sent from his Exchange account, not his POP account, even though the initial calendar invite was sent TO the POP account.

If the user chooses to "edit the response first" he can then choose the POP account and then send the accept response.  In this way the response actually DOES come from the POP account.

Question:  Is there any way to make the "send the response now" option send from the correct account?  In this case POP and not the Exchange account?

Thanks!