This happened a few times. So I am curious why.
For some purpose, I did
tar cf /backup/os.tar / --one-file-system. Then booted into rescue system and remade root filesystem. Then
tar xf /backup/os.tar. And
sendmail could not work. Either was
Watched various logs for a day, I figured out the root cause. All was because of permission was changed for some files. For example,
/usr/lib/dbus-1.0/dbus-daemon-launch-helper was not executable.
/var/lib/sendmail/ belongs to wrong group.