If you can receiv email fine, but cannot send messages through outlook....

Outlook 2007, 2010 and 2013 (and Outlook 2003 SP2 and above)

1. Go to Tools menu > Account settings
2. Select the relevant GreenNet email account, and click the “Change” button (third from the left above that line)
3. Make sure that the “Outgoing mail server (SMTP)” box reads “smtp.gn.apc.org” if this is a GreenNet account. (If you have “smtp.greennet.org.uk” it is worth changing to “smtp.gn.apc.org”.)
4. Click the “More settings…” button to the bottom right.
5. Click the second tab along, “Outgoing server” and ensure “My outgoing server (SMTP) requires authentication” is ticked. Select “Use same settings as my incoming mail server”. “Require secure password authentication” should not be ticked.
6. Click the right-hand tab, “Advanced”.
7. For “Outgoing server (SMTP)”, remove “25” if that is what you have, and first try entering 587. In this case change “Use the following type of encrypted connection” to TLS. Click “OK” and next.
8. Cancel any messages being sent, and try sending again.
9. If you still get an error, you may want to try using “2525” “Outgoing server (SMTP)” without any encryption (not secure), or possibly “465” with SSL encryption (secure).