What are some alternatives?
Hornetsecurity - Hornetsecurity offers you comprehensive, integrated managed e-mail security services.
SpamTitan by TitanHQ - Comprehensive Email Protection. 99.99% Spam Detection. Blocks phishing, viruses, malware and other email threats.
SpamTitan - SpamTitan is an anti-spam email solution specifically developed to meet the demands of modern business. Delivering a spam detection rate over 99.9%
MailArchiva - MailArchiva is an email archiving, email compliance, and ediscovery enterprise software server.
SpamAssassin - Use this program to "Unspam" your mailbox(es).
Symantec Email Security.cloud - Symantec Email Security.cloud provides always-on, inbound and outbound email security against malware, targeted attacks, and advanced threats