If you have multiple email addresses linked to your Outlook client or perhaps have access to some Shared Mailboxes and would like to send an email from one of those accounts, outlook has the capability to change the Sender of an email. However sometimes the option to change the "From" option won't appear straight away and your new email may look something like below:

You can get the "From" selection by clicking on the "Options" button above the new email and then selecting the "From" box that are highlighted below  :

After you have selected this , your new email window should appear like this :

You then just need to click the From button and add the mailbox you wish to send from by using the "Other E-mail Address" option.