As described in this RedHat bug, selinux-policy-2.4.6-255.el5_4.3 (1/7/2010) breaks postfix sendmail.
You'll see messages like:
fatal: username(1002): unable to execute /usr/sbin/postdrop -r: Success
postfix/sendmail[19202]: warning: premature end-of-input on /usr/sbin/postdrop -r while reading input attribute name
in /var/log/maillog.
You have a couple of options for dealing with this issue.
1. You can roll back to an earlier policy version and wait for a patched version to come out.
2. You can update the policy manually as described in the bug.
3. You can temporarily change selinux to permissive mode using "echo 0 > /selinux/enforce" until RHEL produces the patch. Then you can switch to enforcing again using "echo 1 > /selinux/enforce".
TIMTOWDI and all that, so perhaps you can think of others.
