If you have Send As permissions on another recipient object (e.g. mailbox, distribution group) you have the ability to specify that object’s primary SMTP address as your “From” address when sending from Outlook and Outlook Web App (OWA). How you achieve this in OWA is not immediately obvious, so here are some (hopefully helpful) screenshots. The method works for both Exchange 2013 on-premises and Office 365 OWA.
1. In the New Mail pane, click on the three dots to expose additional options. Select “Show From”.
2. With the From field visible, right-click the email address shown and select “Remove”.
3. Click on the drop-down arrow in the From field. From there select the address you which to send from/send as.
4. The address should now appear in the From field. When you send the email it will send from the address shown.
That’s all there is to it. It’s not rocket science, but then it’s not blindingly obvious either.