Configure Exchange 2010 and outlook users with the ability to send email from multiple alias addresses

The users must have multiple SMTP addresses in their mailbox properties. Before doing this, you might need to add the different domain names into the accepted domain. Using email address policies, you can set the user's email addresses. based on the number of accepted domains, the users will have multiple email addresses (if configured so). Out of these multiple email addresses, one will be set as the default SMTP Address.

That is how things are configured currently on the Exchange server, however, what are the steps to configure in Outlook?  I have setup an alias for myself and attempted to walk through this article, as it is similar to what I want to do: https://helpdesk.illinoisstate.edu/kb/1318/How_to_send_from_an_email_alias_in_Outlook_2007_2010/

Difference is, we're not using POP3.  When I setup the new address I get the attached error.  I can still complete the new account setup and select it  in the FROM drop down, but upon sending a test email I receive an NDR:
"You can't send a message on behalf of this user unless you have permission to do so. Please make sure you're sending on behalf of the correct sender, or request the necessary permission. If the problem continues, please contact your helpdesk."
NewAccountError.PNG

Based on the error you are getting sounds like you need to configure the Send As permission.

This article walks you through configuring that. Then repeat what you were doing in Outlook.
http://technet.microsoft.com/en-us/library/bb676368(v=exchg.141).aspx

You can receive emails from different address like user@company1.com or user@company2.com  or user@company3.com.But to send as either of the email addresses is not  what normally people would do. You need to decide what the primary smtp address would be for each user and set it up using email address policy.
The solution that you want can be implemented if its for 1-2 users, but if you want to setup the whole organisation then its going be really hard to manage

We do this in our company a lot. You can enable FROM field in outlook using the Options menu within new email window. (http://www.msoutlook.info/question/121)

You can then type any group email or other email addresses which the user has permission to use (send behalf of will also work, or group email which user is member of).

I personally use it to send email out using our helpdesk group email. I only have to type in the correct email address in from field in outlook. No exchange server setup required for me as I am member of that group.

To be honest the first time I figured this out, I was surprised how easy this is.

I'm not sure that configuring the "Send as" permission will work, as the email address is setup as an alias on my own mailbox.  Perhaps I need to create a new and separate user mailbox? But that could be really messy...

I have enabled the from field in Outlook, but as I said, I get an error when sending from the alias address.  

I see this need growing going forward, so I'd like to implement a solution with minimal manual configuration.  I will check out the utility from Ivasoft, though, a free solution would be ideal :)

Thanks to all for your help on this, looks like the solution is the 3rd party tool or additional mailboxes.  I hope in future releases of Exchange, Microsoft implements a way to address this.