Mail and Exchange 2007 won't allow customizable "From" Header

Question

I use Mac Mail with an Exchange 2007 server. In Preferences > Accounts, under "Account Information," I have two different e-mail addresses (e.g., the address for that exchange server and a gmail address) listed in the "Email address" field. When I create a new message, there is a pull-down menu next to "From:" that is supposed to allow me to choose which of those two e-mail addresses should appear in that field.
However, regardless of which e-mail address I choose, the e-mail message always has the "From:" address for the exchange server. In other words, it won't allow me to customize the "From:" field. This worked properly in the past and I've been using it for awhile, but it quit working when I re-installed Mail. Everything else works perfectly.
Anyone else experiencing this? Any recommendations?
Thanks!

Answer 1

Thanks for your message.
Actually, changing "Reply-To" isn't what I need. I need the proper e-mail address to show in the "from" field; for example, because I need to send messages to a server and those messages need to come from a specific e-mail address.
And, the pull-down message is definitely next to the "From" field whenever a new message is generated.
I actually don't receive messages in Mail for the other two accounts (e.g., I use Gmail's web interface for Gmail messages). I'm simply needing to modify the "From:" field in OUTGOING messages, something that used to work but quit working recently.