This is a problem that I've faced, though on a lesser scale, with one
of my lists; since it's a gay list, I don't think people should
forward messages and potentially out people. But it does happen.
Usually strong words are enough.
If you have the time (and the energy), you could try drastic tactics:
Take a post (or construct one) that you think is likely to be
forwarded from the list. Split the membership into, say ten chunks,
and create ten slightly different versions of the post. Perhaps a
little extra punctuation in different paragraphs. Send it out
manually, then wait and see which version gets posted to the
You'll have narrowed (hopefully!) the culprits down to 10% of the
list. Iterate as necessary.
Another way of checking out some of the ways that a message might be
getting out is to include headers to trigger receipts and
acknowledgements, and compare the hundreds that come back with the
I've periodically done this to verify all the addresses, but it's a
lot of work. If you send an individual message to each person you
might get information back that will reveal whether or not list
messages are being automatically forwarded to a third party from a
particular address. Or you might have to read through 1000 receipts
It's all a lot of work, but I don't see any other way to do it.
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