+PH/02 When tcp-wrappers is called from Exim, it returns only "deny" or "allow".
+ "Deny" causes Exim to reject the incoming connection with a 554 error.
+ Unfortunately, if there is a major crisis, such as a disk failure,
+ tcp-wrappers gives "deny", whereas what one would like would be some
+ kind of temporary error. A kludge has been added to help with this.
+ Before calling hosts_ctl(), errno is set zero. If the result is "deny", a
+ 554 error is used if errno is still zero or contains ENOENT (which occurs
+ if either of the /etc/hosts.{allow,deny} files is missing). Otherwise, a
+ 451 error is used.
+