Replace SpamAssassin with rspamd

I would like to propose replacing SpamAssassin with Rspamd as the default spam filtering solution in Zimbra.

Rspamd offers several advantages over SpamAssassin, including:

1. Better performance and scalability – Rspamd is designed for high throughput and can handle significantly more messages per second than SpamAssassin, making it more suitable for modern mail systems.

2. Advanced filtering capabilities – It includes built-in support for fuzzy checks, neural network-based filtering, and a powerful rule system.

3. Actively maintained and modern architecture – Unlike SpamAssassin, which is aging and has a slower development cycle, Rspamd is actively maintained and offers more modern features out of the box.

Additionally, Rspamd comes with a built-in web interface for monitoring statistics and modifying configuration, which could easily be integrated into the Zimbra Web Panel via a simple link—eliminating the need to develop a new UI from scratch. This is the way it works in the mailcow.

I believe this change would bring Zimbra's spam filtering capabilities up to current standards and offer users a more powerful and user-friendly experience.