Spam problem

I have a client that gets about 3,000 spams per day. It is not coming from
any internal pc, as I've watched it coming into the external global mailbox.
The spam filter on our mailserver is doing a good job, but this many makes
it impossible to find the genuine 'false positives'.

The majority have addresses beginning with an 'a' (e.g. aa.amato@ourdomain,
a1b2l@ourdomain, a.i.a.evans@ourdomain) with spoofed senders.

Does anyone know of any way to trace the origin or some way to fight these
at source?


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