What are some alternatives?
When comparing SpamAssassin and BitNinja.io, 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.
Akamai - Akamai is the leading content delivery network (CDN) services provider for media and software delivery, and cloud security solutions.
Mailwasher - Over 8 million people use MailWasher as their spam filter
CloudFlare - CloudFlare can be used by anyone with a website and their own domain, regardless of your choice in platform
MailScanner - MailScanner is a open source email security system design for Linux-based email gateways.
Altospam - Antispam and antivirus software on Cloud