Ubuntu server sends spam (postfix)

Question

I find out my server is sending a spam. Spam is sent by postfix server. It has large queue of emails, that are going to be sent without my help. I cant understand which script is added these emails to postfix queue.

Now I have these questions:

1. How to determine what script is adding mails to postfix queue?
2. How to clear postfix queue from spam? (all emails are spam, there are no emails sent by me)
3. Why reports are recieved by user123? (user123 - is ubuntu user, not original, changed by security reason)

Report from /var/mail/user123:

From MAILER-DAEMON  Tue Nov 11 04:01:47 2014
Return-Path: <>
X-Original-To: user123@ubuntu
Delivered-To: user123@ubuntu
Received: by ubuntu (Postfix)
        id 8F0D227364; Mon, 10 Nov 2014 15:15:52 -0500 (EST)
Date: Mon, 10 Nov 2014 15:15:52 -0500 (EST)
From: MAILER-DAEMON@ubuntu (Mail Delivery System)
Subject: Undelivered Mail Returned to Sender
To: user123@ubuntu
Auto-Submitted: auto-replied
MIME-Version: 1.0
Content-Type: multipart/report; report-type=delivery-status;
        boundary="C0BE92ECAB.1415650552/ubuntu"
Message-Id: <20141110201552.8F0D227364@ubuntu>

This is a MIME-encapsulated message.

--C0BE92ECAB.1415650552/ubuntu
Content-Description: Notification
Content-Type: text/plain; charset=us-ascii

This is the mail system at host ubuntu.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                   The mail system

<[email protected]>: delivery temporarily suspended: host
    smtp-in.orange.fr[80.12.242.9] refused to talk to me: 550 mwinf5c20 ME
    Adresse IP source bloquee pour incident de spam. Client host blocked for
    spamming issues. OFR006_102 Ref
    http://csi.cloudmark.com/reset-request/?ip=74.218.214.24 [102]

--C0BE92ECAB.1415650552/ubuntu
Content-Description: Delivery report
Content-Type: message/delivery-status

Reporting-MTA: dns; ubuntu
X-Postfix-Queue-ID: C0BE92ECAB
X-Postfix-Sender: rfc822; user123@ubuntu
Arrival-Date: Wed,  5 Nov 2014 13:50:50 -0500 (EST)

Final-Recipient: rfc822; [email protected]
Action: failed
Status: 4.0.0
Diagnostic-Code: X-Postfix; delivery temporarily suspended: host
    smtp-in.orange.fr[80.12.242.9] refused to talk to me: 550 mwinf5c20 ME
    Adresse IP source bloquee pour incident de spam. Client host blocked for
    spamming issues. OFR006_102 Ref
    http://csi.cloudmark.com/reset-request/?ip=74.218.214.24 [102]

--C0BE92ECAB.1415650552/ubuntu
Content-Description: Undelivered Message Headers
Content-Type: text/rfc822-headers

Return-Path: <user123@ubuntu>
Received: by ubuntu (Postfix, from userid 1006)
        id C0BE92ECAB; Wed,  5 Nov 2014 13:50:50 -0500 (EST)
From: =?UTF-8?B?T25seSBDYXNpbm8=?= <[email protected]>
To: "MOIDU88480" <[email protected]>
Subject: =?UTF-8?B?Qm9uam91ciBNT0lEVTg4NDgwLiBWZWdhcyBEYXlzIENhc2lubyAtIExhcyBWZWdhcyBzJ2ludml0ZSBjaGV6IHZvdXMgc3VyIFZlZ2FzIERheSBDYXNpbm8h?=
Content-Type: multipart/mixed; boundary="PHP-mixed-3b3472b0874837cf2218d941eec5b6d8"
Message-Id: <20141105185050.C0BE92ECAB@ubuntu>
Date: Wed,  5 Nov 2014 13:50:50 -0500 (EST)

--C0BE92ECAB.1415650552/ubuntu--

Googling gives no result. My google search queries could be wrong, but I really need to fix this problem.
So any help is appreciated.
If I can provide more useful information please ask it in comments.

P.S. Server is hosting magento and wordpress sites. P.S.S. 74.218.214.24 - is IP of my dedicated server, not original. It was changed in this post due to security reason.

UPDATE Some lines from /var/log/mail.log:

Nov  9 06:40:05 u17135818 postfix/smtp[10428]: 65EDE3C718: to=<[email protected]>, relay=mta6.am0.yahoodns.net[98.136.216.25]:25, delay=7.7, delays=7.4/0/0.19/0.06, dsn=5.7.1, status=bounced (host mta6.am0.yahoodns.net[98.136.216.25] said: 553 5.7.1 [BL21] Connections will not be accepted from 74.218.214.24, because the ip is in Spamhaus's list; see http://postmaster.yahoo.com/550-bl23.html (in reply to MAIL FROM command))
Nov  9 06:40:05 u17135818 postfix/smtp[10428]: 65EDE3C718: lost connection with mta6.am0.yahoodns.net[98.136.216.25] while sending RCPT TO
Nov  9 06:40:05 u17135818 postfix/pickup[10080]: 1338B3ED4A: uid=1006 from=<user123>
Nov  9 06:40:05 u17135818 postfix/cleanup[12998]: 1338B3ED4A: message-id=<20141109114005.1338B3ED4A@ubuntu>
Nov  9 06:40:05 u17135818 postfix/cleanup[13261]: 133D53ED54: message-id=<20141109114005.133D53ED54@ubuntu>
Nov  9 06:40:05 u17135818 postfix/smtp[10424]: DECBB27368: to=<[email protected]>, relay=mx2.hotmail.com[207.46.8.199]:25, delay=9.6, delays=9.3/0.02/0.19/0.06, dsn=5.0.0, status=bounced (host mx2.hotmail.com[207.46.8.199] said: 550 OU-002 (BAY004-MC6F11) Unfortunately, messages from 74.218.214.24 weren't sent. Please contact your Internet service provider since part of their network is on our block list. You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors. (in reply to MAIL FROM command))
Nov  9 06:40:05 u17135818 postfix/smtp[12030]: EFA783D645: to=<[email protected]>, relay=gmail-smtp-in.l.google.com[2607:f8b0:4001:c08::1b]:25, delay=7.3, delays=6.6/0/0.09/0.64, dsn=5.7.1, status=bounced (host gmail-smtp-in.l.google.com[2607:f8b0:4001:c08::1b] said: 550-5.7.1 [2607:f1c0:841:fe00::66:d8fd      12] Our system has detected that 550-5.7.1 this message is likely unsolicited mail. To reduce the amount of spam 550-5.7.1 sent to Gmail, this message has been blocked. Please visit 550-5.7.1 http://support.google.com/mail/bin/answer.py?hl=en&answer=188131 for 550 5.7.1 more information. sd5si10854734igb.33 - gsmtp (in reply to end of DATA command))
...
Nov 11 04:01:54 u17135818 postfix/smtp[17765]: E01792762C: host mx1.free.fr[212.27.48.6] said: 451 too many errors detected from your IP (74.218.214.24), please visit http://postmaster.free.fr/ (in reply to DATA command)
Nov 11 04:01:54 u17135818 postfix/smtp[17797]: 953592B312: host cluster1.eu.messagelabs.com[85.158.143.99] refused to talk to me: 450 Requested action aborted [7.2] 21614, please visit www.messagelabs.com/support for more details about this error message.
Nov 11 04:01:54 u17135818 postfix/qmgr[17712]: C7D883257C: from=<user123@ubuntu>, status=expired, returned to sender
Nov 11 04:01:54 u17135818 postfix/qmgr[17712]: 0799A259AD: removed
Nov 11 04:01:54 u17135818 postfix/qmgr[17712]: 90F4332280: removed
Nov 11 04:01:54 u17135818 postfix/qmgr[17712]: 67B8B2E7C7: from=<user123@ubuntu>, status=expired, returned to sender
Nov 11 04:01:54 u17135818 postfix/qmgr[17712]: 9063532F5D: removed
Nov 11 04:01:54 u17135818 postfix/qmgr[17712]: EE4222A874: removed
Nov 11 04:01:54 u17135818 postfix/smtp[17724]: 61C22360A0: to=<[email protected]>, relay=smtp4.princetonhcs.org[209.123.81.114]:25, delay=381492, delays=381485/5.6/0.59/0, dsn=4.5.0, status=deferred (host smtp4.princetonhcs.org[209.123.81.114] refused to talk to me: 550 5.5.0 74.218.214.24 is blacklisted by FortiGuard. This email from IP  has been rejected. The email message was detected as spam.)
Nov 11 04:01:54 u17135818 postfix/smtp[17800]: 61B3A3AD2C: to=<[email protected]>, relay=none, delay=259892, delays=259884/2.2/5.5/0, dsn=4.4.3, status=deferred (Host or domain name not found. Name service error for name=starbucks.org type=MX: Host not found, try again)
Nov 11 04:01:54 u17135818 postfix/smtp[17787]: CD3312175D: host mx1.free.fr[212.27.48.7] said: 451 too many errors detected from your IP (74.218.214.24), please visit http://postmaster.free.fr/ (in reply to DATA command)
Nov 11 04:01:54 u17135818 postfix/smtp[17819]: 780C624266: to=<[email protected]>, relay=mx1.free.fr[212.27.48.7]:25, conn_use=5, delay=227385, delays=227377/6.5/0.66/0.34, dsn=4.0.0, status=deferred (host mx1.free.fr[212.27.48.7] said: 451 too many errors detected from your IP (74.218.214.24), please visit http://postmaster.free.fr/ (in reply to DATA command))
Nov 11 04:01:54 u17135818 postfix/smtp[17778]: CE12E26756: to=<[email protected]>, relay=smtpz4.laposte.net[194.117.213.1]:25, delay=133031, delays=133023/6.5/0.79/0.27, dsn=5.0.1, status=bounced (host smtpz4.laposte.net[194.117.213.1] said: 501 5.0.1 Emetteur invalide. Invalid Sender. LPN007_405 (in reply to MAIL FROM command))

Answer 1

It looks like one service or software triggering this mails. You can block all outgoing mails frompostfix by using the mail relaying options for external domains, this is possible if you don't want to send any mails from your machine.

You can check the maillog file inside /var/log - that will give the more details, also check the command mailq to see how many mails are pending.

Update:-

Do you allowed any of other people in your network to send mail through your machine ?, then you can suspect that case. Few things I can notice from the log is that -

1. The mail being rejected by the receiver end saying your public IP is flooding mails.

2. If these mails are coming periodically and not from any of other machines in your network, then you have to find out which process or application doing this. For that you have to use the tcpdump and monitor for the TCP packets. From that you can see that, the mail client first pushing the mail to your local postfix server, then that's being forwarded to the target mail server.

This is the way I can see to find out which application sending mails from your computer.

Hope this will help you to figure out the culprit.