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1. In Outlook, select New E-mail.
2. Click the Options tab in the top ribbon, then select From. This will add the From option to any e-mail.
3. Click on the From field and select Other Email Address. Enter the e-mail address of the shared inbox you intend to send the e-mail from then select OK. 
4. Once you have added the shared mailbox e-mail address for the first time, it won't need to be added again. It can be selected from the From field drop down in a new e-mail.
If you are using Exchange Online, please follow these instructions below:
1. Once logged into your account, please select New E-mail in the upper left-hand corner of the screen:
2. Select the Options tab then select the check box that says Show From.
3. Select From and then select other email address...
4. Type in the e-mail address of the shared mailbox that you want to send the e-mail from.
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