Smarthost being ignored

[Simon Powell]

Hey folks,
Had my Kolab 2.2 up and running for quite some time but all of a  
sudden it has decided it wants to ignore the smarthost/relayhost I  
have given it and send mail out directly. I'm on a dynamic IP and so  
this is a big no no, hence why I gave it my ISP's smtp box as a  
smarthost. All was working fine until about two weeks ago. Finally got  
round to looking at it today and am stumped - in the web admin portion  
the option is there for it to use smtp.myisp.com and I can hand roll  
messages thru this server so am not being blocked - indeed the sample  
bouncebacks I get when trying to send mail indicate that it is not  
even trying to use the smarthost, despite the relayhost directive  
being defined correctly in my postfix/main.cf.
Anyone any clues as to why this is maybe happening?
Cheers in advance

Si.