From Bugzilla Helper:
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.0.2) Gecko/20030208
Netscape/7.02

Description of problem:
After installing the sendmail 8.12.8 update, local mail on the system is
bouncing. The bounced message indicates

(reason: 553 5.5.4 <root@localhost.localdomain>... Real domain name
required for sender address)
    (expanded from: root)

Sample instance of a bounced mail from /var/log/maillog in "Additional
Information" section.

This system has always run with sendmail without customizations. Users use
Netscape for email. Local mail is used by Logwatch.

Version-Release number of selected component (if applicable):
sendmail-8.12.8-1.80

How reproducible:
Always

Steps to Reproduce:
1.Run mail, or pine(using for test)
2.To: root (from root), Subj: Test, Body: Test
3.Send mail and exit pine
4.Re-enter pine
    

Actual Results:  Bounced mail present in inbox with reason error code & message
as decribed before.

Expected Results:  Mail should have been delivered and not bounced back.

Additional info:
/var/log/maillog event sample:
Mar 21 13:50:54 mthood sendmail[11383]: h2LIosB6011383:
from=<root@localhost.localdomain>, size=281, class=0, nrcpts=1,
msgid=<Pine.LNX.4.44.0303211350410.11382-100000@localhost.localdomain>,
proto=ESMTP, relay=root@localhost
Mar 21 13:50:54 mthood sendmail[11387]: h2LIosPe011387: ruleset=check_mail,
arg1=<root@localhost.localdomain>, relay=localhost [127.0.0.1] (may be forged),
reject=553 5.5.4 <root@localhost.localdomain>... Real domain name required for
sender address
Mar 21 13:50:54 mthood sendmail[11386]: h2LIosB6011383:
to=<root@localhost.localdomain>, ctladdr=<root@localhost.localdomain> (0/0),
delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=30275, relay=[127.0.0.1]
[127.0.0.1], dsn=5.6.0, stat=Data format error
Mar 21 13:50:54 mthood sendmail[11386]: h2LIosB6011383: h2LIosB5011386: DSN:
Data format error
Mar 21 13:50:54 mthood sendmail[11387]: h2LIosPe011387:
from=<root@localhost.localdomain>, size=281, class=0, nrcpts=0, proto=ESMTP,
daemon=MTA, relay=localhost [127.0.0.1] (may be forged)
Mar 21 13:50:54 mthood sendmail[11387]: h2LIosPf011387: from=<>, size=2314,
class=0, nrcpts=1, msgid=<200303211850.h2LIosB5011386@localhost.localdomain>,
proto=ESMTP, daemon=MTA, relay=localhost [127.0.0.1] (may be forged)
Mar 21 13:50:54 mthood sendmail[11386]: h2LIosB5011386:
to=<root@localhost.localdomain>, delay=00:00:00, xdelay=00:00:00, mailer=relay,
pri=31305, relay=[127.0.0.1], dsn=2.0.0, stat=Sent (h2LIosPf011387 Message
accepted for delivery)
Mar 21 13:50:54 mthood sendmail[11388]: h2LIosPf011387:
to=<root@localhost.localdomain>, delay=00:00:00, xdelay=00:00:00, mailer=local,
pri=32536, dsn=2.0.0, stat=Sent