If you send an email on behalf of someone else, you may want people to respond to that person instead of you. Microsoft Outlook allows you to choose another default Reply Address to cover this situation.
Once you have created your mail, switch to the "Options" tab and then click the "Direct reply to" button. (Depending on your version of Outlook, you may need to click File> Properties instead.)
When the Properties window opens, change the address in "Have the answers been sent to "the box from your address to the address (or addresses, if there is more than one) that you want replies sent.
Click on the "Close" button and then send the record as usual.
When the recipient receives the email and clicks "Reply" or "Reply All", Outlook will automatically fill in the address you have selected. This will not prevent them from changing it to your address (or anyone else's) – just like a regular email – but it will make it obvious that they should respond.