Currently Being ModeratedMay 30, 2013 11:05 PM (in response to starion)
Every message that goes through your server should be logged, and the default log data should give an indication as to the sender and recipient.
The problem may be one of filtering - identifying the spam (signal) from all the legitimate mail traffic (noise). For that you really need some of the spam messages (or, at least, the headers) which will give you several critical pieces of information - it will give you the message ID which can be used to identify the message in your logs, it will give you the client IP address (which may identify the machine that generated the message) and it may lead to the user ID that sent the message (assuming you're enforcing authentication on your server and are not an open relay).
Without the message ID it'll be hard to identify valid vs. spam messages in the logs.