Pieter,
I have come across situations where for some unknown reason, SME / E-Smith cannot / will not deliver mail to particular domains (eg. one client had a situation where mail to everyone works EXCEPT to anyone with a bigpond domain like someone@bigbond.com, someone@bigpond.com.au, someone@bigpond.net.au, etc.) I have tried everything I can think of but the mails just simply will not go. I can ping any of those domains and browse the web pages on those domains, just cannot send mail to them. It is not the ISP as I have other sites on the same ISP with no problems.
The only way to work around the problem was to get SME / E-smith to pass the mail to the ISP's SMTP server for delivery (from SME server manager panel).
Perhaps this might solve your problem.
Kelvin