I found this topic was already discussed but I think there should be something different in the way Zammad create users.
I have too many users automatically created when SPAM mails arrives even when I put filters to ignore mails that do not comes from known domains. So, if the filter works and is not permiting ticket creation why to create the user for that mails that is being ignored for ticket creation?.
Anyway Zammad is a great tool for basics or complex scenarios 
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Thanks!.