Sendmail has a bad habbit of breaking after system updates because it doesn't like a group writeable directory. Also, if you to make it so / is not writable, it prevents you from editing files are not in folders.
However, the DontBlameSendmail option allows certian file checks to be overwritten, including Group Writable Directories. This allows Sendmail to work and not make your harddrive read-only. Sendmail's configuration file is very complex so the use of M4 configuration files is recommend. I'm assuming you don't already have a setup M4 file, and you're just starting from scratch. If you already have a file, read the comment at the bottom of this hint. You'll need the Developer Tools installed, and make sure you log in as an admin and then follow the steps in the second part of this hint...
Fixing Sendmail:
cd /usr/share/sendmail/conf/cf
cp generic-darwin.mc system-darwin.mc
echo "define(`confDONT_BLAME_SENDMAIL', `GroupWritableDirPathSafe')dnl" >> system-darwin.mc[That should be one line]
mv /etc/mail/sendmail.cf /etc/mail/sendmail.cf.apple mv /etc/mail/submit.cf /etc/mail/submit.cf.apple
m4 ../m4/cf.m4 system-darwin.mc > /etc/mail/sendmail.cf m4 ../m4/cf.m4 system-darwin.mc > /etc/mail/submit.cf
sed "s/MAILSERVER=-NO-/MAILSERVER=-YES-/" /etc/hostconfig > /etc/hostconfig.new && mv /etc/hostconfig.new /etc/hostconfig[That should be all on one line]
/System/Library/StartupItems/Sendmail/Sendmail startNow sendmail is running in daemon mode so you can use 127.0.0.1 as an SMTP server.
Trying 0.0.0.0... Connected to 0. Escape character is '^]'. 220 nemesis1.local ESMTP Sendmail 8.12.9/8.12.9; Sat, 3 May 2003 21:15:18 -0400 (EDT)Then you have a working sendmail configuration.
define(`confDONT_BLAME_SENDMAIL', `GroupWritableDirPathSafe')dnlNote 2: Should Sendmail stop working due to a upgrade of sendmail (Apple upgrades Sendmail from time to time), repeat all steps expect for #3 to fix it.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030503213653261