What are some alternatives?
When comparing Rspamd and Symantec Messaging Gateway, you can also consider the following products
SpamAssassin - Use this program to "Unspam" your mailbox(es).
Barracuda Email Security Gateway - Barracuda's Email Security Gateway is a powerful antivirus filter specifically designed to identify and eliminate email-borne threats, stopping hazards before they reach the network.
MailScanner - MailScanner is a open source email security system design for Linux-based email gateways.
Proofpoint Email Security and Protection - Learn about Proofpoint's email security solutions that protect organizations against threats such as email fraud, phishing, spam, bulk email, and viruses.
Proxmox Mail Gateway - Protect mail server against spam, viruses and malware with Proxmox Mail Gateway security for businesses, government, education and non-profit organizations.
Cisco Email Security - Cisco Email Security protects against ransomware, business email compromise, spoofing, and phishing.