Hey there reillytj,
Sounds like it is a problem caused by either firewall/outlook or Nav.
Your answer to dave2312uk question will help us with the firewall section.
- Have you tried uninstalling outlook and reinstalling it
- Have you tried to uninstall Nav and see whether you can send/receive emails.
Next, try send an email and while it is sending, go into command prompt and run the command netstat -b.
This will show what connections are being made and to where, if there is a connection on port 25 to localhost, then it is most likely an antivirus issue.