I'm sending them through outlook. It doesn't seem to make any difference whether the addressees are in the "to" section, or the "cc" section.
My isp is BTInternet/Yahoo. I have reported the problem to them, and was told it was an outlook issue, in their opinion, and that they couldn't help.
I have two accounts on Outlook - my personal BTinternet one and my work one (which is a web-based Dutch isp). the outgoing server for both is the BTinternet one. While the Dutch isp is responsible for my incoming mail, it cannot influence my outgoing mail, beacuse of this fact, so I don't think it is them.
It is actually only my parent company in holland that complain about this problem (that i seem to be causing), so I'm not sure whether other people/companies are also affected. I will try to send something to different groups on both accounts to see if that makes any difference, as I suspect that would be one of your next suggestions!