Eric Schulte <schulte.eric@gmail.com> writes:
> I use orgstruct mode in my message mode, so that I may easily include
> tables and lists when writing and responding to mail.

orgstruct-mode does not help you with tables.  That is orgtbl-mode.

> This week, I've suddenly noticed that when I press meta-RET in message
>mode with orgstruct mode enabled, Org-struct returns the error
>
>   orgstruct-error: This key has no function outside structure elements
>
> However, orgstruct should (and used to) just pass this key combo
> through to the underlying major mode.

orgstruct-mode tries to but does not find an appropriate key binding.

Does this patch help?  (Apply and restart Emacs.)