How to send-on-behalf of another user in Outlook
To Send-on-behalf of another user of a general department email address you must first be given permission to do so. If you are not sure if you have permission to send on behalf of a specific email address, please contact the owner of that email address or the Information Technology Help Desk if you do not know the owner of the email address.
Once you have been given permission to send an email on behalf of someone else:
To ensure replies are directed to your new "from" address you must also set your Reply-To address (otherwise replies will still come back to your address even though the messages appear to be "From" the new address. To set the reply address click on the "Direct Replies To" button in the message toolbar:
How to Send on Behalf of another user in Outlook Web App