Passthrough

Passthrough is a special type of forwarding that allows you to use Mailcast in front of another service already configured to handle email for your domain.

As an example, you might have your domain configured with Google Workspace, and you want to use Mailcast to handle the email for additional alias addresses. You can set up aliases to pass through to your existing email addresses, and then set up additional aliases to forward to other email addresses or call webhooks on your website.

When doing this, Mailcast serves as a proxy in front of your existing email service. The difference between this and normal forwarding, is that Mailcast will need to know the destination servers for your existing email service.
With normal forwarding, the destination servers are looked up using the MX records of the destination domain. With passthrough, Mailcast will use the destination servers you specify.