In your Outlook, you manage several inbox and you need to reply to an email, but not with you regular email.
To send as or on behalf of, follow the quick guideline below:
1. On the Home tab, select New Email.
2. Just above the destinary button, Click the box and select Other E-mail Address... from the dropdown.
Note: The From field is hidden by default.
If the From field is hidden, select Options from within the message you are composing, then select "From".
3. Within the Send From Other E-Mail Addresses window, click From
4. Then, select the name of the person you want to send as or send on behalf of, and click OK.
5. Compose a message as usual and send when ready.