Outlook 2007, 2010 and 2013 (and Outlook 2003 SP2 and above)
Go to Tools menu > Account settings.
Select the relevant GreenNet email account, and click the “Change” button (third from the left above that line)
Make sure that the “Outgoing mail server (SMTP)” box reads “smtp.gn.apc.org” if this is a GreenNet account.