[Exim] spam protection - but local!

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Hi,

i had to realize, that s.o. abuses my server to spam other users on my server.
The email always contains the recipients email address as sender & receiver, which is already quite weird.
However, its also sent localy from a different account, but i cannot trace it back to the account who created these emails.
Any idea, how i can either check the users oubox or check the sent emails to trace em back to the account, that really sent them?
Deactivating local email support is sadly no option, because some vital scripts rely on this support.

Thanks for your help,

Duncan
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