SPAM on my server without username as id ?

[lyes.2k]

Hello,

I have many spam sent from my server without any id or username as usually (over 500 pages).

So how can I find the user who sent SPAM ?

OS: Debian 6 64bit
Mail service: Exim / Dovcot

Thanks in advance for your help.

 

[scsi]

/var/log/exim/mainlog

 

[lyes.2k]

Thanks !!