We are running coexisting Exchange 2007 and Exchange 2013 and in the process of migrating all mailboxes to 2013. I have run into a stumbling block where some users cannot send email from alias while others can. So far the two that cannot have been migrated to 2013. These two use Outlook 2007 via Terminal Server. The users without issue either use Outlook 2010 on stand alone computer, or are still on 2007. What I can figure so far is that the issue can be reproduced using a known working mailbox where sending from an alias has been tested successful by setting it up on the terminal server outlook 2007.