Exchange server and Internet mail - Messages stuck in SMTP out queue

I have a SBS 2003 server with 5 clients running Outlook. I use the POP3 connector to retreive internet mail - no problem at all. I want to use the SMTP connector to send mail out via our ISP. I am told this is OK, the ISP says it is OK, I believe I have configured it OK but it still fails to go. Usually I get an "SMTP sink" failure after it tries to send.
I have configured out ISP's smtp relay as the smtp smart host. (relay.plus.net). The lan is connected via a Dlink ADSL router to the internet. Client PC's can send smtp mail OK directly to the ISP via Outlook Express (used just to test this).
I have turned off all authentication and TSL encryyption etc.
I have looked at smtp logs etc but cannot see anything obvious. just the SMTP sink errors reported when viewing the SMTP connector status in Exchange System Manager.
Can anyone confirm that what I am trying to do is possible (I do not want to use SMTP in both directions if at all possible as I understand this requires hosting configuration changes at the ISP end). I have tried vairous combinations of options in the SMTP connector config such as using HELO or EHLO, ETRN options etc. no luck though.
Any pointers much appreciated.
Kind regards
Richard Scales