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I'm pretty sure you can't do that.<br />
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The &quot;From&quot; field in Outlook is used when you have multiple accounts, not multiple aliases. &nbsp;It's allowing you to select which account to use.<br />
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The only way I know of to change the &quot;From&quot; address is to make that the default SMTP address on the user's mailbox on the Exchange server. &nbsp;However, that will always be the address shown in the &quot;From&quot; field.<br />
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&lt;-=+=-&gt;
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I have setup what you suggested Simon, I need to configure the secondary account to verify the drop down in Outlook. Thanks for your input so far. I will update by end of day hopefully.
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I have added a 2nd authoritative domain in Exchange 2010. Added the alias's to the users I want to receive mail from the new Domain. I am unable to see the drop down in Outlook so the user can pick what email address to send from. When I try and manually type in the address it gives error about not being authorized to send emails from that account. Its the same Active Directory account that has the alias. What changes do I need to make to see the drop down addresses in the FROM field in Outlook. And to allow permission to send from the same Active Directory account. I don't think its anything major but can't find what I am missing on this.
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I have added a 2nd authoritative domain in Exchange 2010. Added the alias's to the users I want to receive mail from the new Domain. I am unable to see the drop down in Outlook so the user can pick what email address to send from. When I try and manually type in the address it gives error about not being authorized to send emails from that account. Its the same Active Directory account that has the alias. What changes do I need to make to see the drop down addresses in the FROM field in Outlook. And to allow permission to send from the same Active Directory account. I don't think its anything major but can't find what I am missing on this.

Unable to send email from an authoritative domain in Exchange 2010

Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.

Exchange

Microsoft Outlook is a personal information manager from Microsoft, available as a part of the Microsoft Office suite. Although often used mainly as an email application, it also includes a calendar, task manager, contact manager, note-taker, journal, and web browser.