(reason: 550 Host unknown) sendmail Solaris 10

Mail won't get sent. Here's output in /var/mail for the user:
The original message was received at Sat, 29 Sep 2007 11:44:08 -0500 (CDT)
from localhost []

   ----- The following addresses had permanent fatal errors -----
    (reason: 550 Host unknown)

   ----- Transcript of session follows -----
550 5.1.2 <user@domainxxx.com>... Host unknown (Name server: mailhost: host not found)

Content-Type: message/delivery-status

Reporting-MTA: dns; hostxxx
Received-From-MTA: DNS; localhost
Arrival-Date: Sat, 29 Sep 2007 11:44:08 -0500 (CDT)

In my /etc/hosts I have:
10.xx.xx.xx      mailhost mail

I can ping both mail and mailhost. I can manually telnet mailhost  on port 25 and send an email. I verified sendmail is active. I even stopped and started it. I can ping a public host like www.novell.com and it replies.
online         11:38:58 svc:/network/smtp:sendmail

I have all defaults in /etc/mail.

Any ideas?
Who is Participating?
Please post your sendmail.mc file here.
Make sure you have
define(`SMART_HOST', `smtp-server.domainxxx.com')
where is a back quote ` in front of SMART_HOST and smtp-server......
jjc_mnAuthor Commented:
Here's my sendmail.mc. If I make changes do I  only have to stop and start the snedmail service or do I need to do more?

#  This is a configuration file for SunOS 5.8 (a.k.a. Solaris 8) and later
#  subsidiary machines.  It has support for local and SMTP mail.  The
#  confFALLBACK_SMARTHOST macro is enabled, which means that messages will
#  be sent to that host (which is set to mailhost.$m [$m is the local domain])
#  if MX records are unavailable.  A short-cut rule is also defined, which
#  says if the recipient host is in the local domain, send to it directly
#  instead of the smart host.
#  If you want to customize this further, copy it to a name appropriate
#  for your environment and do the modifications there.
VERSIONID(`@(#)sendmail.mc      1.11 (Sun) 06/21/04')
define(`confFALLBACK_SMARTHOST', `mailhost$?m.$m$.')dnl
What does:

/usr/sbin/host -t mx domainxx.com


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jjc_mnAuthor Commented:
/usr/sbin/host -t mx domainxx.com
domainxx.com mail is handled by 10 mail.domainxx.com.
jjc_mnAuthor Commented:
This might have an affect. I added a second Ethernet card the other day. and added the 63.xx.xx.xx route. I rebooted after so either the new route or a reboot may have stopped sendmail. Sendmail used to work.

Routing Table: IPv4
  Destination           Gateway           Flags  Ref   Use   Interface
-------------------- -------------------- ----- ----- ------ ---------
63.xx.xx.xx          111.xx.xx.x        UGH         1    125  
10.xx.xx.xx          10.xx.xx.xx          U         1    170  ce0
xx.xx.0.0            xx.xx.xx.xx          U         1    500  ce1            10.xx.xx.xx          U         1      0  ce0
default              10.xx.xx.xx          UG        1   1832              UH       33 682283  lo0
jjc_mnAuthor Commented:
turned out the domain name was removed from the host entry
I.E. was:
10.xx.x.xx    hostname
and it needed to be
10.xx.x.xx    hostname  hotname.domainname.com

did that, restarted sendmail and all works
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