Undocumented change, it seems: sendmail in Slackware 9.1 was built
with tcp_wrappers, unlike previous releases. This means that if you
have "ALL: ALL" in /etc/hosts.deny and specifically allow services
in /etc/hosts.allow, after upgrading to Slackware 9.0 you may find
(like me) that sendmail does not work, and logs messages about
tcpwrappers rejections. You need to add a line like "sendmail: ALL"
to /etc/hosts.allow to get it working.