I've been browsing the web for about an hour without any solid leads so I wanted to see if this rings any bells for anyone.
Environment is Exchange 20013. I have a user with Outlook 2007 who has their 2013 Exchange account configured (ABC.com) as well as a secondary POP3 account (123.com).
The behavior I am seeing is that when the user receives email from other users (all users from what I can tell) in the POP3 account domain (123.com) and then tries to open or forward that email, Outlook will try to send as the original sender.
For example, my firstname.lastname@example.org receives an email from email@example.com. When user tries to forward that email, Outlook tries to send as firstname.lastname@example.org.
Naturally, Outlook errors out with a permissions error (you do not have permission to send on behalf of this user).
I've been looking in account settings and haven't found any leads. Also not user how Exchange is factoring into the equation, but the error looks Exchange related. Also, 123.com is not an accepted domain in the Exchange organization.
Thanks for the help.