I plan on offering email forwarding with paid plans.
In meantime (and for free plans), you should consider:
ForwardEmail.net: a freemium email forwarding service with a free tier that is fully configurable via DNS.
Migadu: a low cost ($19/year as of January 2022) email host that includes unlimited domains.
ImprovMX: another freemium email forwarding service.
Cloudflare: free email forwarding, but only if your domain is using Cloudflare’s name servers. Still in beta as of January 2022.
ForwardMX: paid email forwarding.
Most full-service email providers let you configure additional domains.
Google Workspace: “User Alias Domain”
ProtoMail: “Custom Domains”
Zoho Mail: “Domain Aliases”