In order to send email on behalf of another person, that person must first share their mailbox (Delegate Mailbox) and then you will need to apply for access to their shared mailbox (Send on Behalf). 

Please have the user you wish to Send on Behalf of follow the Delegate access process.

Once given permissions, the process for sending mail from their address is as follows:

Create a new email, and then:

  1. Click the 'From' field
  2. Select the 'Other Email Address' option

A text field will appear in which you can enter the Email address you wish to Send from. Enter the users primary email alias into this field and, provided your permissions have been assigned correctly, your email should be issued from that address.