smtp – My postfix is used for spamming

I need your help.

My postfix is sending spam mails.

I checked my main.cf and it should be fine.
In the following a snippet of my mail.log

Jan  3 22:35:41 mail postfix/pickup(104179): 866352963F01: uid=33 from=<web424794656434@mydomain.net>
Jan  3 22:35:41 mail postfix/cleanup(104027): 866352963F01: message-id=<06760aed1118c62b0c4fb17967316e92@www.mydomain.net>
Jan  3 22:35:41 mail opendkim(3895): 866352963F01: DKIM-Signature field added (s=key1, d=mydomain.net)
Jan  3 22:35:41 mail postfix/qmgr(27451): 866352963F01: from=<web424794656434@mydomain.net>, size=6867, nrcpt=1 (queue active)

Any clue what happen?

Thanks in advance!