Hi Harry,
You should not put your ISP's mail server into Delegate Mail Server. What happens is when mail gets to your domain at your ISP end, it waits there till your SME server picks them up. Then SME finds that you have asked it send all mails to your ISP for processing. Your ISP's mail server will think you are trying to relay mail through them and reject those mails. In your case, Delegate Mail server should be blank.
If you want your SME server to send mail for you when you are away from the LAN, search the forums for an add on 'SMTP after POP' which briefly opens the SMTP port on your external interface after a POP request then closes it again after 10 minutes.
Kelvin