What are some alternatives?
When comparing amavisd-new and Symantec Messaging Gateway, you can also consider the following products
MailScanner - MailScanner is a open source email security system design for Linux-based email gateways.
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.
SpamAssassin - Use this program to "Unspam" your mailbox(es).
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.
Rspamd - Rspamd is an advanced spam filtering system that allows evaluation of messages by a number of rules...
Cisco Email Security - Cisco Email Security protects against ransomware, business email compromise, spoofing, and phishing.