What are some alternatives?
Mailwasher - Over 8 million people use MailWasher as their spam filter
MailScanner - MailScanner is a open source email security system design for Linux-based email gateways.
Spamihilator - The free Anti-Spam-Filter Spamihilator removes more than 98% of spam mails before they even get into your inbox.
Scrollout F1 - Scrollout is a free and easy to use anti-spam email gateway designed for Linux & Windows administrators without advanced email security experience.
SMS Shield - Machine Learning-based SMS spam filter for iOS 11
Proxmox Mail Gateway - Protect mail server against spam, viruses and malware with Proxmox Mail Gateway security for businesses, government, education and non-profit organizations.