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Step 1 - Click 'New Email'
Step 2 - Click the 'From' drop-down menu, then select 'Other Email Address...'
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Step 4 - The Shared Exchange Mailbox name will now populate in the 'From' field
Step 5 - The mailbox name will also be available to select from the drop-down menu
Step 4 6 - Finish typing your message, and then choose Send.