I understand your frustration. While the subsequent messages you are receiving may appear to be from the same sender as the previous message, those messages may actually be originating from a totally different, "sent from" account, and even from a different server, in a different country, than the previous messages.
There are a couple of things you can do to see what's happening here:
Check the HEADERS of the inbound spam messages and compare the origination and routing data. You'll have to check each message and save the data into a text file of some kind. On many desktop clients, you can "edit" the message and pre-pend the header above the actual message data.
Once you have ascertained what the SENDING IP ADDRESS is, check your SMTP LOGS. If you are running SmarterMail 12.X, you can search the logs across multiple days and save a lot of time during the search process. Make certain you have the box for "DISPLAY RELATED TRAFFIC" checked when you search, IE:
Because of modern spammers frequently spread the load of their activities across multiple servers, sometimes in multiple countries, they are much more difficult to catch and filter.
The same tactics break the ability to mark something as spam because, in many cases, subsequent messages are not really being sent from the same e-mail address as prior days.
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