I must have been having a "slow" night, as I just "blanked" in my first response to your post.
I now understand that you set up the mail mirroring via the custom MX section of the panel, so please disregard my earlier confusion.
I have often found that real mailboxes go "live", and begin to work much quicker than forwarded addresses. I do not know why this is, but on many occasions my forwarded addresses continued to bounce for up to a full day after my "real" mailboxes were receiving mail. I am not aware of anything that would stop forwarding-only addresses to not work on mail-mirrored domains, and suggest waiting a bit longer to see if the "go live".
It couldn't hurt to contact support, if the forward-only addresses continue to bounce.
Remember: Domains with custom mx records, junk mail filters or that mail mirror another domain cannot be mirrored.
Again, Please forgive my confusion in my first response