[SWLUG] smtp authentication

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not a linux question per se, but i require knowledgable people :)
my local freecycle group uses yahoo groups which now insists that all mail go 
via an authenticated smtp server, so i set up to use my isp (virgin) server 
and authenticated with my virgin username, that part was fine, but yahoo 
promptly bounced the message back becasue the virgin server had replaced the 
'from' address with my virgin address - which i never ever use to send mail - 
and thus the message to the group came from an address that wasn't registered 
to the group.
is this rewriting of addresses normal when sending via an authenticated smtp 
server? if not then i can look at working around it, if it is then either i 
start using my isp email address or kiss goodbye to the group,

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-- 
'The gods,' he said. 'Imprisoned in a thought. And perhaps they were never 
more than a dream.'
(Sourcery)