Fixed - The bcklog of deferred mail is now clear. (11:55 UTC — Feb 24)
Google is once again rate-limiting messages forwarded from our MX servers. We’re working to resolve this and expect the backlog to be cleared over the next several hours.
As a temporary workaround, customers can route an Opalstack mailbox to their forwarding email address and then use that mailbox as a POP3 account in their Gmail settings.
We will update this post when the backlog of forwarded messages is cleared. (01:31 UTC — Feb 24)