I'm trying to determine if it's possible in Outlook(2010 specifically) to select the from address on a per-message basis if the user has multiple valid SMTP addresses on their Exchange mailbox. This is one of those things that is apparently un-googleable. For bonus points, it would be awesome if you could set the default from address per-contact.
In the message window, switch to the Options tab, then enable "From". This will add a "From" input above the "To" line. You can put any mailbox for which you have Send As permission.
(Picture is from this guide)
It looks to me like the OP is asking to choose between multiple SMTP addresses assigned to an individual user's Exchange mailbox.
You cannot choose from the various SMTP (or other proxy types) set on the user's mailbox object in the Active Directory in Outlook. @Stephen Jennings describes how to choose a different recipient object to send "From", but the default SMTP address for the object you select will be stamped on the outgoing message.
The only way to do what you want would be to create multiple recipient objects in the Directory, assign each the appropriate default "From" address, and grant the appropriate user "Send As" permission to them. Then the user can send "From" the appropriate recipient and the address stamped on the outgoing message will be as-expected.
I'd recommend creating Distribution Group objects for the alternate "From" addresses and making the user a member of the groups (as well as granting Send As permission on the groups to the user) so that they'll receive any replies sent to those addresses.
In general I try to never hand-specify alternate addresses (that is, add addresses not managed by policy) on Exchange recipients and to always use groups to facilitate "alternate" addresses for users. I do this for the reason above, amongst others.
The way I got around this was to set up the user with an another AD/Exchange account and then set the primary SMTP as your second/additional address.
Then in your users original account open Outlook, go to Account Settings > Delegate Access and assign the new Exchange user permission of your Inbox. Add the From field in new messages and then flick between both addresses as needed.
Last part is shown here:
A bit long winded but handy if only needed for a few users.