Sending email on another person's behalf requires actions by two people:
1. One person gives delegate access to another person.
2. The delegate then adds the other person's mailbox to their own profile. After that, the delegate can send email on the other person's behalf.
Each person will need to complete the following steps:
1. Select File > Info > Account Settings > Delegate Access.
2. Select Add. Then type the person's name and press Enter to search for it.
3. Select the person's name, then select Add, and then select OK.
4. Under Delegate Permissions, set these five options to None: Calendar, Tasks, Inbox, Contacts, and Notes. Select OK.
Selecting None for each of these options gives the delegate the basic ability to send messages on your behalf, but nothing more. You can grant the delegate additional permissions (such as the ability to see your calendar or inbox) by selecting Calendar > Author or Inbox > Reviewer, for example.
1. Select File > Account Settings > Account Settings.
2. On the Email tab in the list, select Change, and then select More Settings.
3. On the Advanced tab, under Open these additional mailboxes, select Add, and then enter the mailbox name of the person you want to add to your user profile. Select OK, then select OK again.
4. After completing the process, close and restart Outlook.
With the preparation complete, the delegate can now send email on behalf of the other person:
1. Open the new email message.
2. Above the To line, select From, then select the other person's mailbox. If necessary, select Other email address, then enter that email address, then select it.
(If From isn't visible next to the To button, make it visible by going to the ribbon and selecting Options > From.)
3. Finish composing the email, then select Send.
Recipients of the message will see that it is sent by you on behalf of the other person.
Additional information can be found on Microsoft's Support Page.