IMPORTANT: Do not ignore this email.
The hostname  resolves to . It should resolve to (ip address). Please be sure to correct /etc/hosts as well as the ‘A’ entry in zone file for the domain.
Some are all of these problems can be caused by
/etc/resolv.conf being setup incorrectly. Please check this file if you
believe everything else is [...]

In Linux and Unix operating system, it’s possible to use “last” command to display and show the last logged in (and out) users history, and “lastb” command to show and display all the bad login attempts. The users’ login history is been logged and saved in wtmp file while faild login attempts is been stored [...]

The btmp log keeps track of failed login attempts. I have seen on a default linux setup with logrotate configured where the btmp log is left out of rotation and eventually grows out of hand. So first you want to make sure that the btmp log is rotated using logrotate with the below information.
Log Location:/var/log/btmp, [...]