Why cannot I use one of my email addresses as "Send From" while sending an email from Outlook?

In my email account \ Email Address tab in Exchange 2007 of primary domain1.com, I have the other email addresses @domain2.com which is also an accepted domain in my organization. Besides both domains have mutual AD trust relationship established.
The problem is, when sending an email, if I change the "From" address to use the email @domain2.com I got bounced with the error:
 
Delivery has failed to these recipients or distribution lists:

You are not allowed to send this message because you are trying to send on behalf of another sender without permission to do so. Please verify that you are sending on behalf of the correct sender, or ask your system administrator to help you get the required permission.

Can you help?
CastlewoodAsked:
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IvanSystem EngineerCommented:
Hi,

just to clarify, you have 2 email addresses set for your mailbox? One primary, in form of username@domain1.com, and second one is alias, in form of username@domain2.com?

Regards,
Ivan.
Adam BrownSr Solutions ArchitectCommented:
You can only ever use the ReplyTo or Primary email address to send messages from a single mailbox. In order to send as a second email address, you would need to create a second mailbox and assign the second email address to it, then grant full access permissions to the user's mailbox. There is no work around for this. It's by design.

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CastlewoodAuthor Commented:
Need to use Distribution Group, instead of Contact, so the incoming emails can flow into this user's email account.
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