What are some alternatives?
When comparing Rspamd and Akamai, you can also consider the following products
SpamAssassin - Use this program to "Unspam" your mailbox(es).
CloudFlare - CloudFlare can be used by anyone with a website and their own domain, regardless of your choice in platform
MailScanner - MailScanner is a open source email security system design for Linux-based email gateways.
Amazon CloudFront - Amazon CloudFront is a content delivery web service.
Proxmox Mail Gateway - Protect mail server against spam, viruses and malware with Proxmox Mail Gateway security for businesses, government, education and non-profit organizations.
KeyCDN - KeyCDN is a high-performance Content Delivery Network (CDN). Lowest price globally at $0.04/GB with HTTP/2 Support and free Origin Shield.