Scenario: When sending an email, you want to Send as/Send on behalf as someone else.
1. Log in to your mailbox using Outlook Web App.
2. Click + New mail above the folder list. A new message form will appear in the reading pane.
3. Click the more options menu … , and then click Show from.
4. Select the email address that displays in the From field and delete it.
5. Type the name of the user for which you have permission to send email. If you don’t see the name in of the person you want to send From box, you can click Search contact & directory to search the directory of your organization.
6. Add recipients, a subject, and the contents of the message as you typically do, and then click send Send.
Via Outlook 2013/2010
1. Click on New Email.
2. Click the From box and select Other E-mail Address… from the dropdown. (The From field is hidden click Options from within the message and then select From.)
3. You will now be able to type in or select the email address you wish to send as/send on behalf as.