SpamAssassin - Use this program to "Unspam" your mailbox(es).
Truecaller - Find a person by a name or phone number worldwide for free using Truecaller.
MailScanner - MailScanner is a open source email security system design for Linux-based email gateways.
CallApp - Free Caller ID & Call Blocker app that allows mobile users to block phone calls, identify calls, blacklist unwanted callers and much more.
Rspamd - Rspamd is an advanced spam filtering system that allows evaluation of messages by a number of rules...
CallerSmart - CallerSmart is an application that is basically used for the purpose of looking up mystery phone numbers for free.