How do I send an email on behalf of another user? (Outlook)

Your colleague must grant you permission to send emails on their behalf (How do they do that?).

Using Outlook (this feature is not available in webmail):

• Start a new mail message by clicking New Email
  
• If the From box is not visible:
• Click the Options tab
• Click Show From to make the From field appear
• Click on the From button and select Other E-mail Address...
• Click on the From... button and select the contact from your Global Address List and click OK
  
• Type the rest of the email as you would if you were sending it yourself

Note that while you can select any name from the global address list, you can only send on behalf of those who have given you permission.