There are some problems with the email server...
Someone tried to send an email with multiple recipients in the Bcc field, but I got error messages like:

"unrouteable mail domain "hotmail.com"

-----Original Message-----
From: Mail Delivery System
[mailto:Mailer-Daemon@server.mailserver.com]
Sent: Monday, April 25, 2005 1:49 PM
To: off.office@domainsendingmail.com
Subject: Mail delivery failed: returning message to sender

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed:

smarthead@recipient1.com
SMTP error from remote mailer after RCPT TO:<smarthead@recipient1.com>:
host mailbbox1.recipient1.com [xxx.228.75.76]:
450 Client host rejected: cannot find your hostname, [xx.x.166.50]:
retry timeout exceeded
klipmax@recipient1.com
SMTP error from remote mailer after end of data:
host mxs.mail.ru [xxx.67.23.20]: 550 Message was not accepted -- invalid mailbox. Local mailbox klipmax@recipient2.com is unavailable:
user not found
joe570@recipient3.com
SMTP error from remote mailer after RCPT TO:<joe570@recipient3.com>:
host eudoramail-com.mr.recipient3.com [xxx.36.123.68]:
550 <joe570@recipient3.com>: User unknown
anil@recipient4.net
unrouteable mail domain "recipient4.net"
barum.sk@recipient5.com
unrouteable mail domain "recipient5.com"
recipient6@hotmail.com
unrouteable mail domain "hotmail.com"
ERYAP@recipient7.com
unrouteable mail domain "recipient7.com"
recipient8@hotmail.com
unrouteable mail domain "hotmail.com"
farid2@recipient9.com
unrouteable mail domain "recipient9.com"
Globalqs@aol.com
------ This is a copy of the message, including all the headers. ------

Return-path: <off.office@sendingmaildomain.com>
Received: from [xx.xx.155.123] (helo=MARIUSB)
by server.mailserver.com with esmtpa (Exim 4.50)
id 1DQ17b-00008z-Oe; Mon, 25 Apr 2005 06:47:24 -0400
Return-Receipt-To: "sendingmaildomain.com" <off.office@sendingmaildomain.com>
From: "sendingmaildomain.com" <off.office@sendingmaildomain.com>
To: "sendingmaildomain.com" <off.office@sendingmaildomain.com>
Cc: <vero@sendingmaildomain.com>
Subject: Delaware LLC = 2005. Annual Tax. Important. Do NOT DELETE !!
Date: Mon, 25 Apr 2005 13:47:36 +0300
Message-ID:
<!~!UENERkVCMDkAAQACAAAAAAAAAAAAAAAAABgAAAAAAAAARlhlM1QCP0OTzkpMYBAjEsKAAAAQ
AAAAKokqtuqIOUSbCxjCLVAk/QEAAAAA@offshoresimple.com>
X-Priority: 1 (Highest)
X-MSMail-Priority: High
X-Mailer: Microsoft Office Outlook, Build 11.0.6353
Importance: High
Thread-Index: AcVJhBbenzralxGZRjGwKNUMm3xvCA==
X-Message-Flag: Follow up
X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.2180
Disposition-Notification-To: "sendingmaildomain.com"
<off.office@sendingmaildomain.com>
X-PopBeforeSMTPSenders:
allmailboxes@sendingmaildomain.com,a...maildomain.com


Seemed to happen just after I enabled "Track the origin of messages sent though the mail server by adding the X-Source headers " which I initially had disabled but re-enabled to track some spammers down.

Any work around aside disabling it again?

I have Exim Dictionary ACL, Exim 4.5x, Cpanel 10.x, enabled "Include a list of Pop before SMTP senders in the X-PopBeforeSMTP header when relaying mail" and mailman.

Any help appreciated.