Pre-requisites:  IT must have already granted you "Send As" permissions in Zimbra


Temporary

Open Outlook

Then Click on "New Email"


Click on "From"
Click on "Other Email Address..."
Type in the Email address you wish to "Send As".

click 'ok'.
Permanent Drop-Down

To Add to the drop-down "From" menu

Create a new "Persona" in Zimbra Webmail
Click on the "Zimbra" tab (This becomes available after the ZCO (Zimbra Connector for Outlook) is installed).
Click on "Personas"
Add the account information for the persona that you had created and update the "Reply-To" address.

Click "OK".
Select the account information for which address you would like to send as.