Email Validation Checkers
http://check.no6.biz/ (Free)
Tudor Mateescu
http://check.no6.biz/ (Free)
Here is the URL with lots of blacklist: http://multirbl.valli.org/dnsbl-lookup/ Others: https://mxtoolbox.com/blacklists.aspx
List available entropy If you are not sure, whether you need haveged, run: # cat /proc/sys/kernel/random/entropy_avail This command shows you how much entropy your server has collected. If it is rather low (<1000), you should probably install haveged Resources: https://wiki.archlinux.org/index.php/Haveged https://www.digitalocean.com/community/tutorials/how-to-setup-additional-entropy-for-cloud-servers-using-haveged