i am sending mails from my unix server to my mail id
i used sendmail option
previously it ran successfully
now it is not sending mails
what might be the problem

this is the message i am getting in /var/mail/abcdev file

how to rectify this?

----- The following addresses had permanent fatal errors -----
<abc_SUPPORT_DEV@iop.com>

----- Transcript of session follows -----
550 5.1.2 <ABC_SUPPORT_DEV@iop.com>... Host unknown (Name server: mailhost.svr.iop.net: host not found)

--m0L4wCsT023729.1200891492/fugibar.svr.iop.net
Content-Type: message/delivery-status

Reporting-MTA: dns; fugibar.svr.iop.net
Received-From-MTA: DNS; localhost
Arrival-Date: Sun, 20 Jan 2008 22:50:42 -0600 (EST)

Final-Recipient: RFC822; ABC_SUPPORT_DEV@IOP.com
Action: failed
Status: 5.1.2
Remote-MTA: DNS; mailhost.svr.iop.net
Last-Attempt-Date: Sun, 20 Jan 2008 22:58:12 -0600 (EST)

--m0L4wCsT023729.1200891492/fugibar.svr.iop.net
Content-Type: message/rfc822

Return-Path: <abcdev@fugibar.svr.iop.net>
Received: from fugibar.svr.iop.net (localhost [127.0.0.1])
by fugibar.svr.iop.net (8.13.8+Sun/8.13.8) with ESMTP id m0L4ogsT023727
for <ABC_SUPPORT_DEV@IOP.com>; Sun, 20 Jan 2008 22:50:42 -0600 (CST)
Received: (from abcdev@localhost)
by grampian.svr.bankone.net (8.13.8+Sun/8.13.8/Submit) id m0L4ogIU023726;
Sun, 20 Jan 2008 22:50:42 -0600 (CST)
Date: Sun, 20 Jan 2008 22:50:42 -0600 (CST)
From: APPLN IOP Dev id <abcdev@fugibar.svr.iop.net>
Message-Id: <200801210450.m0L4ogIU023726@fugibar.svr.iop.net>
Content-type: text/html
To: ABC_SUPPORT_DEV@IOP.com
Subject: SUCCESS: abc Report Creation Process for 20080120