The default exim configuration on my server was appending the Sender: header regardless of what it was being set to in the php mail() function.
Please see my comment on the following thread: http://drupal.org/node/285486#comment-2181212

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budda’s picture

budda
Primary language English
Location Cheshire
commented
Status: Active » Closed (cannot reproduce)

I think this is a niche problem best covered on a blog post by somebody maybe?
I don't have Exim to verify what you are saying so trust others will find help from your comment.