What are some alternatives?
When comparing SpamAssassin and Kyte, you can also consider the following products
Scrollout F1 - Scrollout is a free and easy to use anti-spam email gateway designed for Linux & Windows administrators without advanced email security experience.
Callblock - The first app to block telemarketers automatically in iOS
MailScanner - MailScanner is a open source email security system design for Linux-based email gateways.
SpamSieve - Save time by adding powerful spam filtering to the e-mail client on your Mac.
Proxmox Mail Gateway - Protect mail server against spam, viruses and malware with Proxmox Mail Gateway security for businesses, government, education and non-profit organizations.
Truecaller - Find a person by a name or phone number worldwide for free using Truecaller.